Ouranosinc / xclim / 5753863869

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Fix multiindex error on upstream xr (#1450)

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### Pull Request Checklist:
- [x] This PR addresses an already opened issue (for bug fixes /
features)
    - This PR fixes #1417
- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] (If applicable) Documentation has been added / updated (for bug
fixes / features)
- [x] CHANGES.rst has been updated (with summary of main changes)
- [x] Link to issue (:issue:`number`) and pull request (:pull:`number`)
has been added

### What kind of change does this PR introduce?

Overriding a coordinate with a pandas MultiIndex was not working
properly before, but this didn't result in any errors up to a recent
change in xarray's master. When calling `assign_coords` with a
multiindex on a coordinate that already existed, the sub-indexes would
not appear in the DataArray repr. In the most recent xarray master, that
now triggers an error when trying to unstack the multiindex, but only if
dask is used. Before, the unstacking would proceed as normal and the
sub-indexes would magically appear on the result. Dropping the previous
coordinate solves this problem.

The next xarray will implement a new public `xr.Coordinates` class that
should be used to assign coords, instead of a raw `pd.MultiIndex`.
Nonetheless, overriding is still not possible and dropping the previous
coordinate is still needed.

### Does this PR introduce a breaking change?
No.


### Other information:
See pydata/xarray#8039.

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