add measure to yaml indicator description

[juliettelavoie]

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?

• Add measure to yaml indicator description. This is a argument that can be given to properties.

Does this PR introduce a breaking change?

no

Other information:

Without this addition, xscen documentation fails.

[aulemahal]

I added the possibility to skip the validation or to have a custom schema.
So I'll defer the review to someone else.

[Zeitsperre]

This PR addresses a significant bug in some downstream libraries. We'll be releasing a patch (0.48.2) after this is merged.

[coveralls]

coverage: 90.193% (+0.003%) from 90.19%
when pulling aadb084 on fix-yamale-measure
into 43612e3 on master.

[Zeitsperre]

@juliettelavoie Merge when ready!