add calc_zostoga to list of functions that change variable size

[paolap]

calc_zostoga goes from input data on (time, depth, lat, lon) to output defined on "time' only.
Grid size passed to mopper is based on input variables and it is used to decided how many timesteps each file should contain
together with the maximum file size. If a correction is not applied a lot of small files will be created for zostoga.
However, this needs to be balanced with the memory used by process. So while output can easily be stored in one file, it's possible that trying to do so will need more memory than what is usually allocated to one file.