Update translators.kerchunk in preparation for TIFF reader fix

[elyall]

• Tests passing
• Full type hint coverage
• Changes are documented in docs/releases.rst
• New functions/methods are listed in api.rst
• New functionality has documentation

Two small changes:

1. virtual_vars_from_kerchunk_refs now takes in arr_refs instead of refs. This allows a single array to be wrapped.
2. find_groups is spun out. TIFFs can store a Dataset or a DataArray. So I was imagining this could be used for something like:

refs = kerchunk.tiff_to_zarr(urlpath)
is_dataarray = find_groups({"refs": refs}) == []

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (support/v1@3bf3f78). Learn more about missing BASE report.

Additional details and impacted files

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##             support/v1     #297   +/-   ##
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  Coverage              ?   81.86%           
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  Files                 ?       41           
  Lines                 ?     2553           
  Branches              ?        0           
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  Hits                  ?     2090           
  Misses                ?      463           
  Partials              ?        0           

Files with missing lines	Coverage Δ
virtualizarr/translators/kerchunk.py	96.42% <100.00%> (ø)

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