Add data_override to NUOPC cap

[mnlevy1981]

I've talked with @gustavo-marques and @alperaltuntas about being able to use data_override to make one-off changes to incoming forcing fields. For CESM, it would require adding three files to the input data repository for each MOM6 grid:

INPUT/grid_spec.nc  INPUT/ocean_hgrid_180829.nc  INPUT/ocean_mosaic.nc

There are two gotchas to be aware of:

1. For now the INPUT data must be a subdirectory of ${RUNDIR}, but maybe we could ask GFDL to replace the hard-coded INPUT/ with a variable that can be set via input.nml. In the meantime, we could probably modify https://github.com/ESCOMP/FMS_interface to give us that flexibility.
2. These files are generated by https://github.com/NOAA-GFDL/FRE-NCtools, so we'd probably want to make sure those tools are easily available on the NCAR machines.

This is a fairly low priority at the moment, but we don't the idea to be completely forgotten... hence this issue ticket.