Numpydoc Compliance

[Zeitsperre]

Pull Request Checklist:

• [ ] This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• [ ] Tests for the changes have been added (for bug fixes / features)
  • [ ] (If applicable) Documentation has been added / updated (for bug fixes / features)
• [x] CHANGELOG.rst has been updated (with summary of main changes)
  • [x] Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Integrates the numpydoc library for docstring validation.
• Fixes the codespell configuration (previously was not parsing all relevant files).
• Drops Python3.9 coding conventions everywhere.
• Reformats existing docstrings using \* to use * instead.
  • On a related note, disabled flake8's RST210 and RST213 checks that were flagging * symbols in docstrings as unterminated emphasis and strong tags.
• Adds a step to the Indictor.parse_indice that sanitizes the * symbols from Parameters entries.
• Painstakingly modifies all docstrings to be closer to the numpydoc specification.

Does this PR introduce a breaking change?

It should not. Call signatures and docstrings will be slightly changed.

Other information:

https://numpydoc.readthedocs.io/en/latest/validation.html

Except for the custom fields that we use to populate our documentation, the docstrings of xclim are very consistent and virtually compliant with the standard now.

Both xclim.sdba and xclim.core.missing are not evaluated by numpydoc due to major refactoring efforts that are underway (https://github.com/Ouranosinc/xsdba) or needed (#2000).

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[github-actions[bot]]

[!NOTE] It appears that this Pull Request modifies the main.yml workflow.

On inspection, the XCLIM_TESTDATA_BRANCH environment variable is set to the most recent tag (v2024.8.23).

No further action is required.

[Zeitsperre]

@coxipi I just want to update the CHANGELOG one last time, but otherwise, this PR is simply waiting on the fix to support the latest xarray.

Thanks again for your comments!