Add tests to aggregate.py

[RondeauG]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• This PR does not seem to break the templates.
• CHANGELOG.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• Mainly, this adds tests to the lines in aggregate.py that had yet to be tested.
• Bugfix: climatological_op now correctly handles kwargs.
• Bugfix: climatological_op with linregress now correctly handles min_periods
• Removed obsolete code now that we pin xarray >=2023.11.
• Bugfix: datablock_3d did not use the same attributes for the rotated pole when creating the fake dataset.

Does this PR introduce a breaking change?

• method=='interp_centroid' in spatial_mean has been removed without backwards compatibility or staged removal, since it could produce vastly erroneous results.
• Not really breaking since the function was not public, but regrid._get_grid_mapping was moved to spatial.py and made public.

Other information:

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