add coordinate disambiguation with cf_xarray

[chrishavlin]

This adds an optional dependency, cf_xarray, in order to help with coordinate disambiguation. When cf_xarray is installed, the conversion from xarray coordinate name to yt coordinate name will also check cf standard names for the selected fields.

still to do:

• update docs

Closes #48

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.82%. Comparing base (6d87e82) to head (db3c9dc).
Report is 18 commits behind head on main.

❗ Current head db3c9dc differs from pull request most recent head f099d79

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
- Coverage   98.95%   98.82%   -0.14%     
==========================================
  Files          20       15       -5     
  Lines        1922     1273     -649     
==========================================
- Hits         1902     1258     -644     
+ Misses         20       15       -5     

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[chrishavlin]

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