fix: potential temp to temp conversion for MOM6 model_interpolate

[hkershaw-brown]

Description:

All temperature observations were failing the forward operator for MOM6 because model_interpolate was only doing interpolation if the QTY was in the state & QTY_TEMPERATURE is not part of the model state.
MOM6 has QTY_POTENTIAL_TEMPERATURE so this needs converting to QTY_TEMPERATURE

Following the method in the POP model_mod. I haven't generalized the code across both model_mods 1) because the salinity units are different between the models 2) POP has depth of grid points static across the ensemble vs thickness in the state for MOM6, 3) I don't want to mess with the POP model_mod

pseudo code:

pressure = f(depth)
pot_temp = f(QTY_POTENTIAL_TEMPERATURE, QTY_SALINITY, pressure) 

Need to double check the units on pressure (dbars vs bars) and salinity (MSU vs PSU)
Note sure of the best name for the conversion routine "sensible_temp" or "in-situ_temp" or whatever.
Note sure what references need to be in the depth to pressure calculation (plop in the three that are in the pop model_mod?)

Also has fix 360.0_r8 for numbers that were missing _r8.

Fixes issue

fixes #773

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Documentation changes needed?

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.

Tests

model_mod_check interpolate compare to values at grid points for variables that are in the state QTY_POTENTIAL_TEMPERATURE, QTY_SALINITY (in python notebook)
See unit notes above - need to check.

Checklist for merging

• Updated changelog entry
• Documentation updated
• Update conf.py

Checklist for release

• Merge into main
• Create release from the main branch with appropriate tag
• Delete feature-branch

Testing Datasets

• Dataset needed for testing available upon request
• Dataset download instructions included
• No dataset needed

[johnsonbk]

@hkershaw-brown Do you have a testing dataset sitting somewhere on glade?

[hkershaw-brown]

NWA regional:
/glade/derecho/scratch/hkershaw/DART/Crocodile/Enrico/filter_run

[johnsonbk]

Thanks!