The present invention is related to a method for cleaning a water vessel in which water is stored over a period of time. Water is often stored in magazines, pools, containers and similar vessel. Water is also stored in net pens in which fish and other types of marine creatures are farmed. Such water vessels will over time become fouled and need cleaning. That is particularly true for all inner walls of such water vessels. Many such water vessels can be quite large and cleaning them efficiently involves removing the water before cleaning and filling up with new water after the cleaning has been completed. Removing water from the water vessel and then refilling the water vessel after cleaning is time consuming and costly. It is also a process that is not environmentally friendly. In regions where water is rare, it can also be a process that is unsustainable. Furthermore, if the water vessel is used to farm fish or other marine creatures, the fish or other marine creatures must be removed before the water is emptied from the water vessel. During cleaning the fish or other marine creatures must be stored in spare water vessels. That further adds to the time it takes to clean the water vessel and also adds to the cost of the whole process. Some publications describing cleaning of net pens include CN 211793762 U, WO 2017/026899 A1, JPH 105709 A and NO 160753 B. However, none of these publications describe a satisfactory method for cleaning the inner walls of a water vessel. Thus, an object of the present invention has been to provide a method for cleaning a water vessel that removes at least one or some of the problems with currently known methods. It has also been an object to provide a method for cleaning a water vessel that is environmentally friendly and saves water. Another object has been to provide a method for cleaning a water vessel that is more time efficient and reduces the cost involved to clean a water vessel. Another object has been to provide a method for cleaning a water vessel that is particularly suitable for cleaning a net pen for fish farming. These objects are met with a method as defined in claim 1, Further embodiments of the present invention are defined in the dependent claims. Hence, there is provided a method for cleaning a water vessel in which water is stored, the water vessel comprising an inner wall. The method comprises arranging a temporary wall near the inner wall of the water vessel so that the majority of the water in the water vessel is contained by the temporary wall, and thereafter cleaning the inner wall of the water vessel. The term “near” above should be understood as a distance between the inner wall of the water vessel and the temporary wall such that there is a gap present between the inner wall of the water vessel and the temporary wall that is large enough to carry out the cleaning of the inner wall of the water vessel. Further, the term “the majority of the water” should be understood such that as much of the water in the water vessel as possible is arranged in and contained by the temporary wall, but not more than that a gap is present between the inner wall of the water vessel and the temporary wall that is large enough to carry out the cleaning of the inner wall of the vessel. To facilitate the arranging of the temporary wall in the water vessel while water is stored in the water vessel, the temporary wall can be packed up, rolled up or folded up before it is arranged in the water vessel and unpacked, unrolled or unfolded as the temporary wall is arranged in the water vessel or after the temporary wall is arranged in the water vessel. Preferably, the temporary wall is arranged in the water vessel such that there is a gap between the inner wall of the water vessel and the temporary wall. Preferably, the temporary wall is made with substantially the same shape as the water vessel, but with smaller dimensions. Thus, when the temporary wall is filled with water there will automatically be a gap between the inner wall of the water vessel and the temporary wall. Furthermore, the support element or elements which supports the temporary wall and the water contained by the temporary wall, is preferably arranged in a position to help maintain the gap between the temporary wall and the inner wall of the water vessel. The inner wall of the water vessel can be cleaned by using a cleaning agent. Alternatively, the inner wall of the water vessel can be cleaned by physically removing unwanted matter and substances from the inner wall. For that purpose, a scrubbing brush or a similar tool may be used. Obviously, the cleaning of the inner wall of the water vessel may be carried out as a combination of using a chemical cleaning agent and a suitable tool in order to physically remove unwanted matter that is stuck to the inner wall. The water in the gap between the temporary wall and the inner wall of the water vessel may be removed before cleaning takes place. Alternatively, the inner wall of the water vessel may be partially or completely cleaned before the water in the gap between the inner wall of the water vessel and the temporary wall is removed. Thus, the water vessel may be cleaned while there is still water in the gap. The resulting dirty water in the gap may thereafter be removed from the gap before the temporary wall is removed from the water vessel and the water stored within the temporary wall is transferred back to the water vessel. If the cleaning of the water vessel is carried out while there is still water in the gap between the temporary wall and the water vessel, the inner wall of the water vessel is preferably cleaned with a suitable tool, such as a scrubbing machine comprising a scrubbing brush as mentioned above. The temporary wall is preferably watertight so that cleaning does not affect the water contained by the temporary wall. The temporary wall is preferably made of a flexible material, such as tarpaulin or a watertight canvas material, or any other material that is watertight and strong enough to hold the water during cleaning of the water vessel. The temporary wall is preferably removed after the cleaning is finished. Water is preferably transferred from the temporary wall to the gap between the temporary wall and the inner wall of the water vessel through a water transferring device in the temporary wall after the cleaning is finished. Thus, to facilitate the removal of the temporary wall and the transfer of the water contained by the temporary wall during cleaning of the water vessel, the temporary wall is preferably provided with at least one water transferring device. The water transferring device is preferably fastened to the wall of the temporary wall and can be set to allow fluid flow through it and to close for fluid flow through it. The water transferring device preferably comprises an opening device and a removable, or at least partially removable, cover element which covers the opening and prevents water flow through the opening when water is contained by the temporary wall. Alternatively, the water transferring device may comprise a valve device which can be opened and closed for fluid flow. Obviously, if there is fish or other marine creatures in the water contained by the temporary wall during the cleaning of the water vessel, the valve device and/or the opening is preferably large enough to allow the fish and marine creatures to be transferred back to the water vessel together with the water. The water vessel can, for example, be a net pen. Alternatively, the water vessel may of course also be a pool, for example a swimming pool. A non-limiting embodiment of the present invention will now be described with reference to the attached figures, where: Figure 1 shows a water vessel which is filled with water. Figure 2 shows the water vessel in figure 1 where a temporary wall is about to be arranged in the water vessel. Figure 3 shows the water vessel in figures 1 and 2 where the temporary wall shown in figure 2 has been pulled through about half of the distance in the water vessel. Figure 4 shows the water vessel in figures 1-3 where the temporary wall shown in figures 2 and 3 has been pulled through the water in the water vessel and is suspended from a support element on the opposite side of the water vessel. Figures 1-4 show four different steps in the present method for cleaning a water vessel 1 which may be a net pen or a pool, such a swimming pool. Figure 5 shows the water vessel after the temporary wall has been installed and the water in the gap between the water vessel and the temporary wall has been removed. Figures 1-5 shows various steps in the process of installing a temporary wall 5 in a water vessel 1 comprising an inner wall 3. When the inner wall 3 needs to be cleaned, the temporary wall 5 is installed into the water vessel 1 such that a large part of the water is contained within the temporary wall 5 while the inner wall 3 of the water vessel 1 is being cleaned. In normal use, the water vessel 1 is filled with water and after some time the inner wall 3 of the vessel 1 will be fouled so much that the inner wall 3 needs to be cleaned. Today, the normal way to clean the inner wall 3 would be to remove all the water from the water vessel 1 and then clean the inner wall 3. If the water vessel 1 is a net pen for farming of fish or other types of marine creatures, the fish or marine creatures will have to be removed from the water vessel 1 before the water is removed. Thus, during the cleaning of the water vessel 1, the fish or marine creatures must be kept in other storage facilities. According to the present invention, to avoid removing all the water 2 in the water vessel 1, a temporary wall 5 is arranged in the water vessel 1. The temporary wall is arranged in the water vessel such that as much as possible of the water in the water vessel 1 is arranged within the temporary wall 1 Thus, the temporary wall 5 is preferably arranged close to the inner wall 3 of the water vessel 1 as indicated in figures 3 and 4 where the gap 10 between the temporary wall 5 and the inner wall 3 of the water vessel 1 is as narrow as possible while making the cleaning of the inner wall 3 possible. Thereby, most of the water in the water vessel 1 is contained by the temporary wall 5. The temporary wall can be rolled up on a drum 7 in a packed-up state as indicated in figures 2-4, which makes it easy to unroll the temporary wall 5 as it is being arranged in the water vessel 5. The drum may be rollably supported on a drum support 8 so that the drum can be removed after the cleaning process is completed. After the packed up temporary wall 5 is arranged in the water vessel 1, the temporary wall 5 is unpacked and attached to the drum support and/or one or a plurality of support elements arranged around the upper boundary of the water vessel as indicated in figure 5. The temporary wall 5 hanged off from, or attached to, one or more suitable attachment devices 11. The temporary wall 5 will now have assumed its shape, which may be substantially the same as the shape of the shape of the water vessel 1. As explained above, the water in the gap 10 between the water vessel 1 and the temporary wall 5 is preferably removed before cleaning takes place, but cleaning may also take place while the water is still present in the gap 10 and the dirty water may be removed after the cleaning is partially or completely completed. Consequently, cleaning of the inner wall 3 may start while water is present in the gap 10, and finished after the water in the gap 10 has been removed. As indicated in figure 5, the temporary wall 5 is preferably provided with a water transferring device 12 for transfer of water through the wall of the temporary wall 5. The water transferring device 12 may comprise a through-going opening in the temporary wall 5 and a cover device 13 which makes the water transferring device 12 watertight in its closed position and allows water, together with fish or marine creatures present in the water, to flow through the opening of the water transferring device 12 in its open position. The cover device 13 is especially useful when temporary wall 5 is removed after the cleaning of the water vessel 1 is finished. The cover device 13 of the water transferring device 12 is opened and the water within the temporary wall 5, and any fish or marine creatures present in the water, is transferred through the opening of the water transferring device 12 and into the gap 10 between the inner wall 3 of the water vessel 1 and the temporary wall 5. When the gap 10 has been filled with water, it is easy to remove the temporary wall 5 from the water vessel 1. Thus, the water transferring device 12 facilitates the removal of the temporary wall 5 after the cleaning has been finished. The cover device 13 is preferably remotely operated from outside the water vessel 1, for example by one or more lines that pull the cover device 13 away from the opining of the water transferring device 12. The temporary wall 5 can be made of tarpaulin or some other material that is preferably watertight in order to isolate the water contained by the temporary wall 5 during cleaning thereby avoiding fouling of that water. The temporary wall 5 can provided with a temporary wall edge 6 which can be pulled through the water as indicated with arrow A in figure 3 when the temporary wall 5 is arranged in the water vessel 5. The temporary wall edge 6 can be attached to a support element 9 as indicated in figures 4 and 5, possibly with the use of an attachment device 11 as indicated in figure 5. Reference numbers: 1. Water vessel 2. Water (in the water vessel and/or the temporary wall) 3. Inner wall (of the water vessel) 4. Water vessel support element (for water vessel) 5. Temporary wall 6. Temporary wall edge 7. Drum (on which the temporary wall is rolled up on) 8. Drum support 9. Support element (to which temporary wall edge can be attached) 10. Gap (between inner wall of water vessel and temporary wall) 11. Attachment device 12. Water transferring device 13. Cover device