Remove zarr dependency

[konstntokas]

• Removed the dependency on zarr. As part of this change,
  xcube_resampling.grid_mapping.cfconv.add_spatial_ref has been removed, as it
  only loosely fit the scope of the xcube-resampling repository.

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/
• Changes documented in CHANGES.md
• CI checks pass
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.64%. Comparing base (5e06d6d) to head (48c4aa2).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
- Coverage   98.65%   98.64%   -0.02%     
==========================================
  Files          20       20              
  Lines        2009     1990      -19     
==========================================
- Hits         1982     1963      -19     
  Misses         27       27              

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