Add cf conventions helper

[banesullivan]

Resolves #15

I feel this should be double-checked by someone more familiar with CF conventions and how that accessor works.

Also, this should be tested against more datasets

cc @rabernat, any feedback here? The premise is that the new function added here can attempt to use cf_xarray to get the XYZ coordinate array names when the user does not specify names.

In future work, this can be used to address #19

[banesullivan]

This just feels wrong

[dcherian]

I agree that is quite ugly.

Alternatives are

x = da.axes["X"][0] if "X" in da.cf else None

or in _get_array:

# if cf_xarray exists
return da.cf[key]  # this falls back to usually xarray stuff if "key" is not interpretable using cf conventions

[banesullivan]

Great! thank you for the suggestions

[codecov-commenter]

Codecov Report

Merging #31 (949d7cc) into main (3231ba8) will increase coverage by 0.63%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   90.73%   91.36%   +0.63%     
==========================================
  Files           8        9       +1     
  Lines         205      220      +15     
==========================================
+ Hits          186      201      +15     
  Misses         19       19              

Impacted Files	Coverage Δ
pvxarray/accessor.py	96.29% <100.00%> (+0.46%)	⬆️
pvxarray/cf.py	100.00% <100.00%> (ø)

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[TomNicholas]

cc'ing @dcherian as a cf-xarray pro