Refactor / add component schemas

[jhamman]

This PR reorganizes the xarray-schema package. A few new features are included (e.g. to_json) but it is mostly a refactor that includes:

• Split source code into separate modules
• component schemas for individual parts of the xarray data model
• parameterized tests

[codecov[bot]]

Codecov Report

Merging #7 (91b2967) into main (2b4b4de) will increase coverage by 11.95%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             main        #7       +/-   ##
============================================
+ Coverage   88.04%   100.00%   +11.95%     
============================================
  Files           2         6        +4     
  Lines          92       258      +166     
============================================
+ Hits           81       258      +177     
+ Misses         11         0       -11     

Impacted Files	Coverage Δ
xarray_schema/__init__.py	100.00% <100.00%> (ø)
xarray_schema/base.py	100.00% <100.00%> (ø)
xarray_schema/components.py	100.00% <100.00%> (ø)
xarray_schema/dataarray.py	100.00% <100.00%> (ø)
xarray_schema/dataset.py	100.00% <100.00%> (ø)
xarray_schema/types.py	100.00% <100.00%> (ø)

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