Update XRImage to create missing 'bands' coordinate when dimension is present

[djhoese]

This was created due to a conversation with @gerritholl about some test datasets in Satpy. The summary is that the test datasets had a "bands" dimension but no data_arr.coords["bands"] entry. Once a call to .squeeze was removed (it had been masking this issue), the enhancement/stretch methods in XRImage were breaking because xarray was defaulting data_arr.coords["bands"] to an integer array of 0. XRImage expects a list of strings (ex. ["R", "G", "B"].

• Closes #xxxx (remove if there is no corresponding issue, which should only be the case for minor changes)
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Passes git diff origin/master **/*py | flake8 --diff (remove if you did not edit any Python files)
• Fully documented (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.77%. Comparing base (d5b3e23) to head (2b4c10d).
⚠️ Report is 51 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
+ Coverage   91.63%   91.77%   +0.13%     
==========================================
  Files          11       12       +1     
  Lines        4114     4135      +21     
==========================================
+ Hits         3770     3795      +25     
+ Misses        344      340       -4     

Flag	Coverage Δ
unittests	91.77% <100.00%> (+0.13%)	⬆️

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[coveralls]

coverage: 92.021% (+0.1%) from 91.881%
when pulling 2b4c10d on djhoese:bugfix-no-bands-coord
into a54a9b2 on pytroll:main.

[pnuu]

LGTM

[mraspaud]

LGTM. I just have one request: update the title of the PR, as I got wrong expectations when reading it vs what is implemented. Proposal for a new title:
"Update XRImage to create missing 'bands' coordinate when dimension is present"