Testing Data Tagging

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR addresses comments made in [PyOpenSci] Reviewer #2 comments #1335
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGES.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?

• Adds some checks when running pytest to ensure that xclim-testdata branches used to test against xclim are compatible.
• Sets the main.yml CI workflow to use tagged commits from xclim-testdata
• Adds a new CI workflow to check whether the tag found in main.yml is consistent with the latest tag in xclim-testdata, and posts comments and/or fails the build if there is disagreement. This is only triggered if changes are made to main.yml
• Updates the documentation on how best to update xclim-testdata.

Does this PR introduce a breaking change?

Yes. XCLIM_TESTDATA_BRANCH now points to tagged testing data commits on CI.

Other information:

[github-actions]

Note
It appears that this PR modifies the XCLIM_TESTDATA_BRANCH environment variable to the most recent tag (v0.41.0).

No further action is required.

[aulemahal]

This looks good, thanks!