Add bilinear regridding support

[stephenworsley]

Adds support for bilinear regridding for the unstructured regridders.

[codecov[bot]]

Codecov Report

Merging #148 (845b6a1) into main (baa1030) will decrease coverage by 0.05%. The diff coverage is 99.38%.

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
- Coverage   99.56%   99.51%   -0.06%     
==========================================
  Files          27       27              
  Lines        2073     2469     +396     
==========================================
+ Hits         2064     2457     +393     
- Misses          9       12       +3     

Impacted Files	Coverage Δ
esmf_regrid/experimental/unstructured_regrid.py	97.36% <90.00%> (-2.64%)	:arrow_down:
esmf_regrid/experimental/unstructured_scheme.py	99.11% <97.10%> (-0.89%)	:arrow_down:
esmf_regrid/_esmf_sdo.py	97.16% <100.00%> (+0.20%)	:arrow_up:
esmf_regrid/esmf_regridder.py	95.08% <100.00%> (+0.53%)	:arrow_up:
esmf_regrid/experimental/io.py	100.00% <100.00%> (ø)
...regrid/tests/unit/esmf_regridder/test_Regridder.py	100.00% <100.00%> (ø)
.../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	100.00% <100.00%> (ø)
...ntal/unstructured_scheme/test__cube_to_GridInfo.py	100.00% <100.00%> (ø)
... and 5 more

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[trexfeathers]

@stephenworsley #162 might solve the CI failures (unless you think the regressions are genuine)?