TITLE: “FEEDING SYSTEM FOR FEEDING A WEB OF LABELING MATERIAL FOR A LABELING MACHINE AND RELATED LABELLING METHOD” ***** TECHNICAL FIELD The present invention relates to a feeding system for feeding a web of labeling material for a labeling machine configured to apply labels obtained from the aforesaid web to containers suitable for containing a pourable product, preferably of food type. The present invention also relates to a method of labeling containers suitable for containing a pourable product, preferably of food type. STATE OF THE ART Labeling machines are known commonly used to prepare, transport and apply labels to containers, in particular bottles, flasks, jars or the like, made of plastic or glass, suitable to contain, or destined to be filled with, a pourable product, preferably of food type. The use of glued labels, obtained from a web of labeling material initially wound in the form of a continuous strip on one or more reels, is particularly widespread. In detail, this web is cut into portions of the same size, to which glue is applied. The glue is applied using gluing means, for example rollers, spray or injection systems, or the like. The labels thus obtained are then transferred and glued onto the outer lateral surface of the respective containers. Another particularly widespread type of labeling provides for the production of sleeve labels, produced from a web of heat-shrink film initially wound in the form of a continuous strip on one or more reels; these sleeve labels are applied with a certain slack on the respective containers and then heated in an oven to obtain shrinking and perfect adhesion thereof to the lateral surfaces of the containers. This type of label does not require the use of glue. A further type of labeling known as PSL (pressure sensitive label) provides for the production of self- adhesive labels initially arranged on a backing web from which they must then be separated to be glued directly on the respective containers. In detail, by passing the backing web through a deflector plate that causes said web to pass through a very tight angle, the various labels separate from the backing web and attach directly to the containers. Regardless of the type of label used, a labeling machine typically comprises: - a carousel rotatable around a vertical axis and configured to convey a succession of containers along a horizontal labeling path in the form of an arc of circle; - an input station, in which the containers to be labeled are fed to the carousel; - an output station, in which the labeled containers leave the carousel; and - one or more labeling modules arranged peripherally relative to the carousel and configured to feed respective pluralities of labels to the carousel itself at an application station, in order to apply these labels to the respective containers. Generally, the labeling module comprises: - a feeding system for unwinding the web from the reels of labeling material, typically supported by corresponding shafts mounted rotatable on the labeling module; - a plurality of support rollers for supporting and guiding the web unwound from the reel along a feeding path; - a cutting station at which the labels are sequentially separated from the web; and - a label transfer device, configured to receive, retain and advance each label previously separated and feed this label to the carousel at the application station. In particular, in the case of glued labels, the labeling module comprises a cutting device (for example a knife or a blade) configured to repeatedly cut, at the cutting station, the web in order to obtain therefrom a sequence of single portions of labeling material defining the labels. In the case of sleeve labels, the labels are wound in a tube around respective forming spindles advanced by the carousel and carrying at the top the containers to be labeled; the labels are then welded at the overlapping ends; at this point, the spindles are withdrawn inside the base of the carousel to allow the containers to be inserted with slack inside the respective sleeve labels; subsequently, the containers with the slack labels are sent to the heating oven to obtain heat-shrinking of the labels and perfect adhesion thereof to said containers. Typically, the transfer device is defined by a vacuum drum configured to receive the previously cut labels, to retain them by suction and, after a rotation through a given angle around its axis, to release these labels at the application station, so that they are applied to the respective containers or to forming spindles. In the case of labels that provide for the use of glue, the labeling module further comprises at least one gluing roller arranged substantially tangent to the vacuum drum to spread glue at least on the ends of each single label. Instead, in the case of PSL labeling, a separation station is present instead of the cutting station defined by the aforesaid deflector plate, which also defines the transfer device, and the labeling module does not comprise any cutting or gluing device. Typically, the feeding system comprises: - a feed roller rotatable around a first axis and actuatable in rotation to control unwinding of the web by gripping on an outer lateral surface thereof; and - a pinch roller rotatable around a second axis, generally parallel to the first axis, and opposed in pressure against the feed roller, so that the outer lateral surface thereof presses against the outer lateral surface of the feed roller, with interposition of the web being unwound. In detail, the feed roller is active, i.e., motorized around a first axis, to control the aforesaid unwinding, while the pinch roller is passive, i.e. rotates idle around the second axis driven by the feed roller, and presses against the feed roller so as to define a counter- pressure roller for this latter and allow correct unwinding of the web from the relative reel, and the feeding thereof along the aforementioned feeding path. Although this is functionally valid, the Applicant has observed how the feeding system described above is susceptible to further improvements. In particular, the Applicant has observed how this feeding system is subject, over time, to a deterioration in its performance. This deterioration can be caused by wear of the pinch roller: in fact, the pinch roller is generally made of elastomeric material, in order to provide the friction necessary for gripping of the web; prolonged dynamic contact with the feed roller and with the web causes consumption of the pinch roller, which determines loss of grip on the web and sliding thereof, resulting in non-nominal feeding along the labeling module, which can cause non-nominal cutting of the labels and non-nominal application thereof to the containers. When the position deviation of the web relative to the nominal positioning becomes excessive, the machine must be stopped and the pinch roller replaced. Deterioration of performance can also be caused by a build-up of residues (such as pigments, paints, powders deriving from labeling material and non-labeling material) on the pinch roller. Also in this case there will be a variation in the gripping force on the web, which can cause sliding or stretching thereof along the direction of feed, thus determining the aforesaid position deviation. When the position deviation of the web relative to the nominal position becomes excessive, the machine must be stopped and the pinch roller replaced. SUBJECT MATTER AND SUMMARY OF THE INVENTION The object of the present invention is to produce a feeding system for a labeling machine, and a related labeling method, which are highly reliable and have a limited cost, and which allow at least some of the drawbacks specified above and related to prior art systems and methods to be overcome. According to the invention, this object is achieved by a feeding system and by a related labeling method as claimed in the appended independent claims. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, some non-limiting preferred embodiments thereof are described below, purely by way of example and with the aid of the accompanying drawings, wherein: - Fig. 1 is a schematic top view, with parts removed for clarity, of a labeling machine comprising a feeding system produced according to the present invention; - Fig. 2 is a schematic side view, on an enlarged scale and with parts removed for clarity, of an embodiment of the feeding system of Fig. 1; - Fig. 3 is a schematic side view, on an enlarged scale and with parts removed for clarity, of an alternative embodiment of the feeding system of Fig. 1; and - Fig. 4 is a schematic top view, on an enlarged scale and with parts removed for clarity, of the feeding system of Fig. 1. DETAILED DESCRIPTION With reference to the accompanying figures, 1 indicates, as a whole, a labeling machine configured to apply labels 2 obtained from a web 3 of labeling material to containers 4 suitable for containing a pourable product, preferably of food type (such as water, beer, wine, carbonated beverages, fruit juices, tea, milk or the like). For the sake of simplicity, reference will be made in the following to a labeling machine 1 operating with labels 2 of the aforesaid glued label type, to which the present description will refer, without any loss of generality. However, what is described and claimed in the following is equally applicable in the case of labels of the sleeve type or of the PSL type described above. Preferably, the containers 2 are defined by bottles or flasks, made of plastic or glass. The machine 1 comprises: - a carousel 5 rotatable around an axis Z, preferably vertical, and configured to convey a succession of containers 4 to be labeled along a preferably horizontal labeling path in the shape of arc of circle; - an input station (not visible), in which the containers 4 to be labeled are fed to the carousel 5; - an output station (not visible), in which the labeled containers 4 leave the carousel 5; and - a labeling module 6 arranged peripherally relative to the carousel 5 and configured to prepare, transport and feed a plurality of labels 2 to the carousel 5 at an application station A, in order to apply these labels 2 to the respective containers 4. Preferably, the web 3 is initially wound in the form of a continuous strip on a reel 3a. In particular, the labeling module 6 comprises: - a storage portion including at least one support shaft 7 mounted rotatable to the module 6 for supporting one said reel 3a at a time; - a feeding system 8 for progressively unwinding the web 3 from the reel 3a; - a plurality of support rollers 10 (only one of which is illustrated) for supporting and guiding, in use, the web 3 along a feeding path P; - a cutting device 11 (of known type and not described in detail) arranged along the feeding path P downstream of the feeding system 8 and configured to repeatedly cut the web 3 and obtain a sequence of labels 2 therefrom; and - a transfer device, for example a vacuum drum 12 of known type, configured to receive, retain and advance each label 2 and feed this label 2 to the carousel 5 at the application station A. The vacuum drum 12 is configured to receive the labels 2, to retain them by means of suction and, after a rotation by a given angle around its axis, to release these labels at the application station A, so that they are applied to the respective containers 4. Moreover, the vacuum drum is configured to space the labels received apart from one another so as to adapt to the pitch of the containers 4. According to this preferred embodiment, the labeling module 6 comprises, furthermore, at least one gluing roller 13 arranged substantially tangent to the vacuum drum 12 upstream of the application station A, to spread glue at least on the ends of each single label 2 before applying it to the respective container 4. As is visible in Fig. 1, and more in detail in Figs. 2, 3 and 4, the feeding system 8 comprises: - a feed roller 14 rotatable around a first axis X, having a first outer lateral surface 14a for supporting the web 3, and actuatable in rotation (around the axis X) to advance the web 3 along the feeding path P; and - a pinch roller 15 rotatable around a second axis Y, conveniently parallel to the axis X, and having a second outer lateral surface 15a opposable in pressure against the first outer lateral surface 14a with interposition of the web 3 for pressing the web 3 against the feed roller 14 and keeping it in gripping on the first outer lateral surface 14a. In practice, the pinch roller 15 is radially opposed to the feed roller 14 to help this latter in unwinding the web 3 from the reel 3a and in advancing the web 3 along the feeding path P. Preferably, the feed roller 14 is active, i.e. it is motorized to rotate around the axis X, while the pinch roller 15 is passive, i.e. it rotates idle, in use, around the axis Y driven (through friction) by the feed roller 14. The pinch roller 15 presses, in use, against the feed roller 14 to keep the web 3 in gripping on the feed roller 14 and in this way allow correct unwinding of the web 3 from the reel 3a, and feeding thereof along the path P. Preferably, the feed roller 14 is made of a rigid metal material, for example of steel. Preferably, the pinch roller 15 is made of an elastomeric material, for example rubber, in order to provide the necessary friction. In an alternative embodiment, the feed roller 14 is made of elastomeric material and the pinch roller 15 is made of rigid metal material. It is clear how the roller made of elastomeric material (i.e., in this preferred embodiment, the pinch roller 15) is subject, over time, to wear (i.e. to consumption) or to the accumulation of residues (for example pigments, paints, powders deriving from the labeling material and the like) on the outer surface 15a thereof. Therefore, it is possible to distinguish: - an initial condition of the pinch roller 15, in which the pinch roller 15 is substantially immaculate, i.e., not yet worn or consumed and therefore still has the original radius, or in which there are no accumulations of residues on the outer surface 15a of the pinch roller 15; and - a deteriorated condition of the pinch roller 15, in which the pinch roller 15 is worn, i.e., consumed and has a smaller radius relative to the original radius, or in which there are accumulations of residues on the outer surface 15a which increase (albeit slightly) the radius of the pinch roller 15. According to the invention, the feeding system 8 further comprises: - a sensor 16, 17, 18 configured to automatically detect a quantity correlated to a current condition of a first roller defined by one of the feed roller 14 and the pinch roller 15, and to generate a signal correlated to the detection of said quantity; - a control unit (not illustrated) configured to receive said signal from the sensor 16, 17, 18 and to automatically control a web grip compensation action on a second roller as a function of the detected quantity and of the current condition of the first roller correlated thereto, the second roller being defined by said one of the feed roller 14 and the pinch roller 15 or by the other of the feed roller 14 and the pinch roller 15. According to this preferred embodiment, the first roller, or “detected roller”, is defined by the pinch roller 15. The aforesaid current condition correlated to the quantity to be detected is thus a current condition, or state, of the pinch roller 15. Again according to this preferred embodiment, also the second roller, or “compensated roller”, is defined by the pinch roller 15. The aforesaid web grip compensation action is thus carried out on the same pinch roller 15, as a function of the current condition determined by means of detection of the aforesaid quantity. In other words, the sensor 16, 17, 18 is suitable, by means of detection of the aforesaid quantity, to determine whether or not this current condition corresponds to the aforesaid deteriorated condition. With reference to Fig. 2, in an embodiment the feeding system 8 comprises a sensor, for example an encoder 16 associated with the pinch roller 15, configured to detect a variation of the angular speed around the axis Y of the pinch roller 15. More precisely, a consumption of the pinch roller 15, or an accumulation of residues on the surface 15a, causes a variation in the radius of said pinch roller 15. When the variation in radius becomes appreciable, the angular speed of the pinch roller 15, which is driven in rotation by the feed roller 14, starts to vary. According to the invention, the control unit is configured to correlate the variation of angular speed detected by the encoder 16 at the current condition of the pinch roller 15, and to thus control the compensation action on the pinch roller 15 as a function of the detected angular speed value. Therefore, the aforesaid quantity is in this case defined by the variation of angular speed or by the angular speed. With reference to Fig. 3, in another embodiment the feeding system 8 comprises a sensor 17 configured to directly detect a variation in the radius or diameter of the pinch roller 15. For example, the sensor 17 comprises a laser sensor, preferably a laser probe, arranged at a predetermined distance from the second outer lateral surface 15a and configured to locally detect a variation of said distance, thus deriving said variation in the radius or diameter of the pinch roller 15. According to the invention, the control unit is configured to correlate the variation in the radius or diameter detected by the laser probe 17 to the current condition of the pinch roller 15, and to control the compensation action on the pinch roller 15 as a function of the detected radius or diameter. Therefore, the aforesaid quantity is in this case defined by the variation in radius or diameter or by the radius or diameter of the pinch roller 15. This configuration is particularly advantageous relative to use of the encoder 16 in the case in which the consumption of the, or the accumulation of residues on the, pinch roller 15 involves only a longitudinal portion of the surface 15a and not the whole surface 15a, as shown in Fig. 3. This can occur in particular in the case of a web 3 of limited width: in this case, wear or accumulation will be localized in this longitudinal portion, and the angular speed might be only slightly influenced, as the remaining portions of the pinch roller 15 would still have the original radius. With reference to Fig. 4, in a further embodiment the system comprises a sensor, for example an optical sensor 18, configured to detect a sliding of the web 3 on the feed roller 14. Preferably, sliding is detected as follows: the optical sensor 18 detects, in use, a linear distance traveled by the web 3 in a time interval, for example detecting reference elements suitably arranged on the web 3 at a preset distance from one another (for example, notches or bands); simultaneously, the control unit: measures a peripheral distance traveled by the feed roller 14, at the surface 14a, in said time interval; compares the measured peripheral distance with the linear distance detected by the optical sensor 18; and calculates sliding of the web 3 as a function of said comparison. As stated above, the aforesaid consumption or accumulation of residues on the surface 15a causes a variation in the radius of said pinch roller 15. When the variation in radius becomes appreciable, the radial distance between the surface 14a of the feed roller 14 and the surface 15a of the pinch roller 15 varies. For example, in the case of consumption, it increases. In this case, the gripping force exerted by the pinch roller 15 on the web 3 against the feed roller 14 decreases. Consequently, the web 3 could slide on the feed roller 14. Vice versa, an accumulation of residues on the surface 15a would increase the gripping force, with consequent stretching of the web 3. After determining the sliding of the web 3, according to the invention, the control unit is configured to correlate the detected sliding of the web 3 to the current condition of the pinch roller 15, and to control the compensation action on the pinch roller 15 as a function of the detected sliding of the web 3. Therefore, in this case the aforesaid quantity is defined by the sliding of the web 3 on the feed roller 14. This configuration is particularly effective for determining a current condition different from the initial condition regardless of the type of web 3 (or label 2) to be treated, in practice correlating the condition to the gripping force. In an embodiment, not illustrated, the system 8 could comprise the encoder 16, the laser probe 17 and the optical sensor 18. In this case, the control unit determines the current condition of the pinch roller 15 as a function of all the quantities detected by these sensors. Thanks to this configuration, the degree of adaptability and flexibility of the system 8 and of the machine 1 to different formats of web 3 (or of labels 2) and containers 4 is increased. The methods with which the aforesaid web grip compensation action is carried out will be described below. Advantageously, the feeding system 8 comprises an actuator 20, preferably of pneumatic type, configured to control a radial displacement of the pinch roller 15 away from or towards the feed roller 14, so as to vary the linear distance between the first axis X and the second axis Y and so as to vary the pressure of the surface 15a on the surface 14a. Alternatively, the actuator 20 could be of the electric, electromagnetic, hydraulic or oleodynamic type. The Applicant has observed how the actuator 20 of pneumatic type is particularly effective in controlling the aforesaid distance, and hence the aforesaid pressure, in a simple, precise and inexpensive way. According to the invention, the control unit is configured to control the actuator 20 as a function of the current condition of the pinch roller 15 determined according to one of the methods indicated above, thus defining the compensation action of the gripping force on the web 3. For example, in the case in which the wear of the pinch roller 15 causes a decrease in the radius thereof, the gripping force on the web 3 will decrease accordingly. At this point, the sensor 16, 17 or 18, or all three sensors or a combination thereof, will detect the relative quantity. The control unit will determine, as a function of this quantity, the current condition of the pinch roller 15, which will correspond to a deteriorated condition thereof, different from the initial condition. At this point, the control unit will automatically control the actuator 20 to move the pinch roller 15 towards the feed roller 14, re-establishing the nominal gripping force and thus carrying out a compensation action on the gripping force. Conveniently, the feeding system 8 comprises a cleaner device 21 arranged adjacent to the surface 15a and controllable to remove residues from this surface 15a. According to the invention, the control unit is configured to control the cleaner device 21 as a function of the current condition of the pinch roller 15 determined according to one of the methods indicated above, thereby defining the compensation action of the gripping force on the web 3. For example, the accumulation of residues and/or dirt on the surface 15a can cause an excessive pressure on the feed roller 14 and on the web 3 being gripped, which can result in stretching of the web 3. The control unit can at this point, as a function of the current condition of the pinch roller 15, control the cleaner device 21 so that the residues are removed, thus re-establishing the nominal cleanliness, and the nominal radius, of the pinch roller 15. Conveniently, the control unit can also control a reverse movement of the pinch roller 15 by controlling the actuator 20, alternatively or simultaneously to controlling the cleaner device 21. The cleaner device 21 could be any device for cleaning the surface 15a, for example a brush or a scraper or a laser cleaner. From the above, it is clear how the current condition is defined by a degree of consumption of the surface 15a or by a degree of cleanliness of the surface 15a determined by the accumulation of residues and/or dirt thereon. It is clear how the feeding system 8, and the labeling machine 1, allow implementation of a method of labeling containers suitable for containing a pourable product by applying labels obtained from a web of labeling material, the method comprising the steps of: a) conveying the containers along a labeling path; b) feeding the web along a feeding path; c) repeatedly cutting the web to obtain a sequence of labels therefrom; d) applying each label to a container; the step b) of feeding includes: - supporting the web on a first outer lateral surface of a feed roller; - actuating in rotation the feed roller to advance the web along the feeding path; - opposing a pinch roller in pressure against the feed roller with interposition of the web, so that a second outer lateral surface of the pinch roller keeps the web gripping on the first outer lateral surface during the step b) of feeding; The method further comprises the steps of: e) detecting a quantity correlated to a current condition of the pinch roller; f) generating a signal correlated to the detection of said quantity; g) automatically controlling a web grip compensation action on the pinch roller as a function of the detected quantity and of the current condition of the pinch roller correlated thereto. Preferably, the step e) of detecting includes: - detecting a variation of angular speed of the pinch roller and/or a variation in radius or diameter of the pinch roller and/or a sliding of the web on the feed roller; - correlating said variation of angular speed and/or said variation in radius or diameter and/or said sliding of the web to said condition of the pinch roller, to determine said current condition of the pinch roller. The step g) of controlling comprises controlling said compensation action as a function of the determined current condition. Preferably, the step g) of controlling comprises: - moving the pinch roller towards or away from the feed roller as a function of said determined current condition, so as to vary the pressure of the second outer lateral surface on the first outer lateral surface; or - controlling a cleaner device to remove residues from the second outer lateral surface, as a function of said determined current condition. In an embodiment, the step g) of controlling can comprise informing a user of a trend over time of the current condition of the pinch roller, or controlling a deteriorated condition alarm. From an examination of the characteristics of the feeding system 8 and of the labeling method produced according to the present invention the advantages that can be obtained therewith are evident. In particular, thanks to the compensation action automatically performed on the pinch roller 15 as a function of the determination of the current condition thereof, by detection of the relative quantity by the sensors 16, 17 and/or 18, it is possible to obtain an automatic adaptation of the system 8, and hence of the machine 1, to the current condition of the pinch roller 15, so as to ensure continuity of the labeling process regardless of the fact that the current condition of the pinch roller 15 passes progressively from the initial condition to the deteriorated condition. This makes it possible to avoid frequent unwanted machine downtimes and to reduce maintenance operations. Furthermore, in the case in which the compensation action comprises the control of an alarm (e.g. sending an alarm signal to a user), the system 8 according to the invention reduces the risks of delayed intervention by a user, or allows future maintenance to be scheduled. It is clear that modifications and variants can be made to the feeding system 8 and to the labeling method described and illustrated here without departing from the scope defined by the claims. In particular, the aforesaid first roller could be defined by the feed roller 14, and the second roller could be defined by the pinch roller 15. This is particularly advantageous if the feed roller 14 is made of elastomeric material and the pinch roller is made of rigid metal material. In fact, in this case, it is the feed roller 14 that is subject to consumption (wear) and to the accumulation of residues and/or dirt on the surface 14a. Moreover, also the second roller could be defined by the feed roller 14. In fact, the actuator 20 could be associated with the feed roller 14 to control the radial position thereof relative to the pinch roller 15; likewise, the cleaner device 21 could be associated with the feed roller 14, to control the degree of cleanliness thereof it could be associated with the feed roller 14.