Add diagnostic: water flux into sea water

[jkrasting]

Add the total water flux into sea water diagnostic. This is the net mass flux going into the ocean, i.e. the sum from all contributions.

[StephenGriffies]

This diagnostic is available via the CMOR field "wfo". I do not recall when this diagnostic was added. But it works, and is now enabled in the OM4 test case diag_table.MOM6.