MITgcm salinity units

[hkershaw-brown]

Discussion today at standup that there are converters that we do not have in DART (collaborators - I am not sure who) that output PSU (g/g). This is a pain, but so as to not break their code, namelist option for ROMS_rutgers & MITgcm to have model_interpolate give MSU or PSU for salinity.

Punting on MITgcm ocean since it has a converter in the model directory: https://github.com/NCAR/DART/blob/main/models/MITgcm_ocean/create_ocean_obs.f90
and with observation converter in this model module: https://github.com/NCAR/DART/blob/main/models/MITgcm_ocean/dart_MITocean_mod.f90
seems to be hfradar. Will move this MITgcm units to another issue

Originally posted by @hkershaw-brown in #1003