Drop build_reduction_data and refactor the reduce module

[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?

• xs.reduce_ensemble will now call xclim.ensembles.create_ensemble and xclim.ensembles.make_critera if required.

Does this PR introduce a breaking change?

• The xscen.reduce module has been abandoned.
  • build_reduction_data has been made redundant by xclim.ensembles.make_critera and will be removed in a future release.
  • xscen.reduce.reduce_ensemble has been moved to xscen.ensembles.reduce_ensemble, as a module was no longer necessary.

Other information:

[RondeauG]

@aulemahal @juliettelavoie I put this as a draft, because this is what I'm proposing:

1. You review the changes as they are here.
2. Once we're happy with them, I move reduce_ensemble to ensembles.py and we fully abandon reduce.py. --> This would mess up GitHub's track changes, which is why I'd like to have the first step.
3. Alongside 2, I fix the documentation and add tests.

[aulemahal]

I agree with those changes!

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