Add nearest neighbour regridding

[stephenworsley]

Creates the minimal version of nearest neighbour regridding described in #213.

This also incidentally fixes a slight error in the refactoring where ESMFBilinear didn't have a use_src_mask and use_tgt_mask keyword.

[codecov[bot]]

Codecov Report

Merging #266 (5abe277) into main (1690171) will increase coverage by 0.05%. The diff coverage is 99.76%.

@@            Coverage Diff             @@
##             main     #266      +/-   ##
==========================================
+ Coverage   98.73%   98.79%   +0.05%     
==========================================
  Files          30       33       +3     
  Lines        3323     3563     +240     
==========================================
+ Hits         3281     3520     +239     
- Misses         42       43       +1     

Impacted Files	Coverage Δ
esmf_regrid/experimental/unstructured_scheme.py	93.93% <ø> (ø)
...sts/unit/schemes/test_ESMFAreaWeightedRegridder.py	100.00% <ø> (ø)
...d/tests/unit/schemes/test_ESMFBilinearRegridder.py	100.00% <ø> (ø)
...f_regrid/tests/unit/schemes/test__ESMFRegridder.py	100.00% <ø> (ø)
esmf_regrid/schemes.py	96.04% <96.42%> (-0.05%)	:arrow_down:
esmf_regrid/esmf_regridder.py	95.58% <100.00%> (+0.13%)	:arrow_up:
.../tests/unit/experimental/io/test_round_tripping.py	100.00% <100.00%> (ø)
...nstructured_scheme/test_GridToMeshESMFRegridder.py	100.00% <100.00%> (ø)
...nstructured_scheme/test_MeshToGridESMFRegridder.py	97.11% <100.00%> (+0.05%)	:arrow_up:
..._scheme/test_regrid_rectilinear_to_unstructured.py	100.00% <100.00%> (ø)
... and 6 more

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