Generalizing MITgcm read in to enabele read in of SPARC

[AaronDavidSchneider]

Topic:

This PR tracks the changes that were performed to make gcmt ready for SPARC/MITgcm.
Thank you for your comments and help Vivien Parmentier and Nishil Mehta.

Improvements:

• atm_p0 is the reference pressure for Theta in MITgcm and is now used as such
• Rs can be defaulted to atm_kappa * atm_cp, where the default of atm_kappa is 2/7. This logic is now implemented.
• rename readin to mitgcm_readin
• proper renaming of variable names from cubedsphere names to gcmt names to be less dependend on cubedsphere.

Todo:

• fix tests (see xarray master #61 for failing xarray rest)
• SPARC tests

[codecov]

Codecov Report

Merging #60 (614c8e1) into main (47e33a6) will increase coverage by 0.28%.
Report is 1 commits behind head on main.
The diff coverage is 91.52%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #60      +/-   ##
==========================================
+ Coverage   78.30%   78.58%   +0.28%     
==========================================
  Files          14       14              
  Lines        1549     1560      +11     
==========================================
+ Hits         1213     1226      +13     
+ Misses        336      334       -2     

Flag	Coverage Δ
unittests	78.58% <91.52%> (+0.28%)	⬆️

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Files	Coverage Δ
gcm_toolkit/core/const.py	100.00% <ø> (ø)
gcm_toolkit/core/units.py	100.00% <100.00%> (ø)
gcm_toolkit/mitgcm/__init__.py	100.00% <ø> (ø)
gcm_toolkit/utils/read_and_write.py	95.09% <100.00%> (ø)
gcm_toolkit/mitgcm/utils.py	94.11% <96.66%> (ø)
gcm_toolkit/mitgcm/read_in.py	94.00% <94.00%> (ø)
gcm_toolkit/utils/manipulations.py	91.39% <81.25%> (ø)