Adjust notebooks to xclim-testdata changes

[Zeitsperre]

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.
• 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?

• Adds the branch name to the xclim.testing.open_dataset calls
• Cleans up a few comments within the Jupyter code blocks

Does this PR introduce a breaking change?

No. This fixes an issue arising from changes to the testing data.

Other information:

The next release of xclim will have modified xclim.testing utilities. Though, this only really affects a few code blocks, it's likely that these changes will need to be revisited eventually.

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[Zeitsperre]

@sarahclaude @juliettelavoie Once this is in, I'll merge the cookiecutter update and the remaining automated PRs should close themselves

[sarahclaude]

Looks good! Thanks Trevor