Tests for spatial.py, remove old 'region'

[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.
• 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?

• Tests for the functions in xs.spatial
• Small bugfix in creep_weights for an edge case where the grid is very small, n is high, and mode=wrap.
• Refactored subset to separate into multiple smaller functions.

Does this PR introduce a breaking change?

• For 2D grids, the grid resolution estimation will now take the maximum difference, instead of the difference of the first indexes. The impact should be minimal.
• Remove old (deprecated) instances of region
• Remove the deprecated xs.extract.clisops_subset()