agrif fresh water budget (fwb) fixes for branch_4.2

As proposed in #78 (closed), the agrif fixes for the fresh water budget correction (fwb/=0) are relevant and should be introduced as well in branch_4.2 code.

The cherry pick from the development branch is rather complex due to the large set of changes already occurred previous months, so this MR addresses the implementation of the bugfixes specifically for the 4.2 code.