[WIP] Update lat and lon variable names in cmor_mixer and cmor_helper

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>critical bits of feedback from @zappalaja</issue_title>
<issue_description>> - Made small changes to cmor_mixer py and cmor_helper py to allow 'lat' and 'lon' variable names instead of 'latitude' and 'longitude' and to allow ‘cartesian_axis’ as an axis option.

Originally written by @zappalaja elsewhere

this should be more than doable</issue_description>

Comments on the Issue (you are @copilot in this section)

@ilaflott crucial bit 1: https://github.com/NOAA-GFDL/fre-cli/commit/9b7e0c0bbd60e42480d5599c838c64518f004870

crucial bit 2: NOAA-GFDL/fre-cli@f0eaa9a</comment_new>

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.83%. Comparing base (2f6d320) to head (09b6c76).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #152   +/-   ##
=======================================
  Coverage   94.83%   94.83%           
=======================================
  Files          11       11           
  Lines        1336     1336           
=======================================
  Hits         1267     1267           
  Misses         69       69           

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