SkyCoord Conversions

[calgray]

Added explicit dataset conversion functions for working with astropy SkyCoord objects.

• skycoord_to_dataset(skycoord, coords)
• dataset_to_skycoord(ds)

SkyCoord objects are still not Dataset-like and can't be used and automatically serialised within DataTree, but conversion is now reliable to handle any SkyCoord value.

Additional useful serialisation functions have also been added and organised into submodules in the coordinates package, but these may change in future.

• Closes SkyCoord to Dataset conversions #8
• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.63352% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.67%. Comparing base (ec7aacd) to head (6648a9a).

Files with missing lines	Patch %	Lines
astropy_xarray/coordinates/frame.py	88.07%	7 Missing and 6 partials ⚠️
...ropy_xarray/tests/test_visibility_serialization.py	84.12%	8 Missing and 2 partials ⚠️
astropy_xarray/time_compat.py	73.33%	7 Missing and 1 partial ⚠️
astropy_xarray/conversion.py	91.42%	1 Missing and 2 partials ⚠️
astropy_xarray/coordinates/representation.py	89.47%	2 Missing ⚠️
astropy_xarray/formatting.py	0.00%	1 Missing and 1 partial ⚠️
astropy_xarray/accessors.py	96.00%	1 Missing ⚠️
astropy_xarray/coordinates/sky_coord.py	97.61%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
- Coverage   93.16%   90.67%   -2.50%     
==========================================
  Files          14       27      +13     
  Lines        1653     2187     +534     
  Branches      158      225      +67     
==========================================
+ Hits         1540     1983     +443     
- Misses         74      152      +78     
- Partials       39       52      +13     

Flag	Coverage Δ
unittests	90.67% <92.63%> (-2.50%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
astropy_xarray/__init__.py	81.81% <100.00%> (+1.81%)	⬆️
astropy_xarray/coordinates/__init__.py	100.00% <100.00%> (ø)
astropy_xarray/coordinates/core.py	100.00% <100.00%> (ø)
astropy_xarray/tests/test_accessors_coords.py	100.00% <100.00%> (ø)
astropy_xarray/tests/test_accessors_log.py	100.00% <100.00%> (ø)
astropy_xarray/tests/test_accessors_time.py	100.00% <100.00%> (ø)
astropy_xarray/tests/test_frame_conversion.py	100.00% <100.00%> (ø)
astropy_xarray/tests/test_sky_coord.py	100.00% <100.00%> (ø)
astropy_xarray/tests/test_time.py	100.00% <100.00%> (ø)
astropy_xarray/accessors.py	85.67% <96.00%> (-8.62%)	⬇️
... and 7 more

... and 2 files with indirect coverage changes

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