Fix robustness fractions and categories attributes

[aulemahal]

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)
• 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?

• This is a simple PR that fixes a problem with overwritten attributes on coordinates when calling robustness_fractions and robustness_categories.

Does this PR introduce a breaking change?

No

Other information:

[Zeitsperre]

Not sure what's going on but LGTM

[aulemahal]

héhé sorry, it's kinda undocumented...

robustness_fractions seems to drop the attributes of the source data is some cases. I explicitly add them back at the end of the function.

Same thing for robustness_categories, but here I understood what was going one. I first initialize the inputs from a copy of the source to ensure further in-place changes are not propagated back to the source. I also inverted the xr.where order because xarray takes the attributes from the second argument which was an integer in the previous code, leading to the attributes being lost.