Allow dataset to be rectified with decreasing x_coords

[konstntokas]

• Added support for rectifying datasets with decreasing x-coordinate in
  xcube_resampling.rectify_dataset.

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

❌ Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.80%. Comparing base (9383992) to head (48d4e14).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
xcube_resampling/reproject.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
- Coverage   98.84%   98.80%   -0.05%     
==========================================
  Files          20       20              
  Lines        1986     2003      +17     
==========================================
+ Hits         1963     1979      +16     
- Misses         23       24       +1     

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