Weighted arrays in ensemble statistics

[RondeauG]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • While not directly related to Helper Function to show results by global warming levels #1014 , projections by warming level will need weighted arrays when multiple RCPs/SSPs are involved.
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• HISTORY.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added
• The relevant author information has been added to .zenodo.json

What kind of change does this PR introduce?

• weights argument added to ensembles.ensemble_percentiles, ensembles.ensemble_mean_std_max_min & ensembles.change_significance

Does this PR introduce a breaking change?

DataArrayWeighted.quantile() requires the latest xarray release (v.2022.06.0)

Other information:

[coveralls]

Coverage decreased (-0.005%) to 91.38% when pulling 391d2da on ens-weights into 768538f on master.

[aulemahal]

Looks good and doesn't seem to interfere much with the non-weighted version.

However, if a specific xarray version is needed, we need to update the requirements.

[aulemahal]

Two minor comments, but this looks good to go!