Fix messy chunks on multivar within sdba.adjustment.PrincipalComponents.train

[j-haacker]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Chunks on multivar changed within sdba.adjustment.PrincipalComponents.train leading to error #2105
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGELOG.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?

• it resets the chunks on the stacked variables in "multivar" while doing a PCA

Does this PR introduce a breaking change?

No.

Other information:

n/a

[github-actions]

Warning

This Pull Request modifies the SDBA module.
The SDBA module is gradually being deprecated in favour of the xsdba package.
Any changes to xclim.sdba should be made in https://github.com/Ouranosinc/xsdba

[j-haacker]

This pull request needs PR #2104.

[j-haacker]

As noted in #2104 (review) I will move the issue and PR to xsdba.