Define XarraySource.from_stac() for more convenient creation of an XarraySource from a STAC Item or ItemCollection

[AdeelH]

Overview

This PR moves most of the XarraySource creation code from XarraySourceConfig to XarraySource.from_stac(), which allows an XarraySource to be created directly from a STAC Item or ItemCollection without having to define an XarraySourceConfig. It also removes the not-useful num_channels_raw argument from XarraySource.__init__().

Checklist

• Added unit tests, if applicable
• Updated documentation, if applicable
• Added needs-backport label if the change should be back-ported to the previous release
• PR has a name that won't get you publicly shamed for vagueness

Notes

N/A

Testing Instructions

N/A

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (e6510d7) 89.75% compared to head (bd9884a) 89.69%.

Files	Patch %	Lines
...tervision/core/data/raster_source/xarray_source.py	70.83%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2061      +/-   ##
==========================================
- Coverage   89.75%   89.69%   -0.07%     
==========================================
  Files         196      196              
  Lines        9735     9736       +1     
==========================================
- Hits         8738     8733       -5     
- Misses        997     1003       +6     

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