Add docstrings

[stephenworsley]

Add docstrings which were still missing.

[codecov]

Codecov Report

Merging #83 (eb1dc52) into main (aa74e97) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #83   +/-   ##
=======================================
  Coverage   98.82%   98.82%           
=======================================
  Files          14       14           
  Lines         678      678           
=======================================
  Hits          670      670           
  Misses          8        8           

Impacted Files	Coverage Δ
esmf_regrid/esmf_regridder.py	94.11% <ø> (ø)

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[jamesp]

See if you can fix the linting on this and we're good to go 👍