allow single field for load_grid

[chrishavlin]

Closes #43

[chrishavlin]

pre-commit.ci run

[chrishavlin]

pre-commit.ci autofix

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (de5f482) 98.67% compared to head (c3d5318) 98.68%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #67   +/-   ##
=======================================
  Coverage   98.67%   98.68%           
=======================================
  Files          14       14           
  Lines        1132     1138    +6     
=======================================
+ Hits         1117     1123    +6     
  Misses         15       15           

Files	Coverage Δ
yt_xarray/accessor/accessor.py	96.79% <100.00%> (+0.03%)	⬆️
yt_xarray/tests/test_accesor.py	100.00% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.