Xarray entrypoint

[aurghs]

• Add the external interface as a plugin for xarray.

• Fix circular dependency using xarray by removing from .xarray_to_grib import to_grib in the init.

• Add some test

• passed black

[FussyDuck]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #203 (a481ac1) into stable/0.9.9.x (de89716) will decrease coverage by 42.14%.
The diff coverage is 4.59%.

@@                 Coverage Diff                 @@
##           stable/0.9.9.x     #203       +/-   ##
===================================================
- Coverage           96.53%   54.38%   -42.15%     
===================================================
  Files                  23       25        +2     
  Lines                1615     1699       +84     
  Branches              191      196        +5     
===================================================
- Hits                 1559      924      -635     
- Misses                 34      761      +727     
+ Partials               22       14        -8     

Impacted Files	Coverage Δ
cfgrib/__init__.py	100.00% <ø> (ø)
cfgrib/xarray_entrypoint.py	0.00% <0.00%> (ø)
tests/test_50_xarray_plugin.py	16.66% <16.66%> (ø)
cf2cdm/cfunits.py	0.00% <0.00%> (-100.00%)	⬇️
cf2cdm/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
cf2cdm/cfcoords.py	0.00% <0.00%> (-100.00%)	⬇️
cf2cdm/datamodels.py	0.00% <0.00%> (-100.00%)	⬇️
tests/test_40_xarray_store.py	4.54% <0.00%> (-95.46%)	⬇️
tests/test_20_cfcoords.py	5.95% <0.00%> (-94.05%)	⬇️
tests/test_50_xarray_getitem.py	7.31% <0.00%> (-92.69%)	⬇️
... and 13 more

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