feat: interpolate nearest spatial

[OpheliaMiralles]

Description

• Add spatial interpolation method to nearest neighbors for missing values
• Comment out a check in xarray preventing single spatial coordinate (e.g. station, cell)
• Adapt complement to other cases than cutout
• Add possibility to leave nans in complement if the nearest points are too far / same for interpolate nearest

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Issue Number

Mostly personal usecases, but I do believe a spatial interpolation was needed (and fixes to complement result from investigation with station data which cannot be interpolated onto a full grid).

Code Compatibility

• I have performed a self-review of my code

Code Performance and Testing

• I have added tests that prove my fix is effective or that my feature works
• I ran the complete Pytest test suite locally, and they pass

Dependencies

• I have ensured that the code is still pip-installable after the changes and runs
• I have tested that new dependencies themselves are pip-installable.

Documentation

• My code follows the style guidelines of this project
• I have updated the documentation and docstrings to reflect the changes
• I have added comments to my code, particularly in hard-to-understand areas

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📚 Documentation preview 📚: https://anemoi-datasets--260.org.readthedocs.build/en/260/