Do not return <this-array> instances

[aulemahal]

Small fix for #179.

Modified _get_with_standard_name so that when used on a DataArray, it cannot return itself (the <this-array> variable in the temp dataset). This match the previous behavior (I think?) and fits the returned error message : ds.air.cf['air_temperature'] fails because 'air_temperature' doesn't appear in ds.air.cf. But ds.lat.cf['latitude'] works, since 'latitude' appears in ds.lat.cf.

Also added a minimal test.

[codecov-io]

Codecov Report

Merging #180 (638b38d) into main (af85a2a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #180   +/-   ##
=======================================
  Coverage   96.09%   96.10%           
=======================================
  Files          11       11           
  Lines        1356     1359    +3     
=======================================
+ Hits         1303     1306    +3     
  Misses         53       53           

Flag	Coverage Δ
unittests	96.10% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
cf_xarray/accessor.py	94.90% <100.00%> (ø)
cf_xarray/tests/test_accessor.py	100.00% <100.00%> (ø)

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[dcherian]

Great! merging. Can you test your downstream packages to make sure there are not other issues.