Add Christmas Day indices, add bivariate_count_occurrences

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Add White Christmas and Perfect Christmas indicators #2029
• 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?

• Adds two new indices and indicators:
  • holiday_snow_days → units = "days"
  • holiday_snow_and_snowfall_days → units = "days"
• Added bivariate_count_occurrences to generic indices.

Does this PR introduce a breaking change?

No.

Other information:

prsn and snd data has been added to the xclim-testdata repository: Ouranosinc/xclim-testdata#30

[github-actions]

Note

It appears that this Pull Request modifies the main.yml workflow.

On inspection, the XCLIM_TESTDATA_BRANCH environment variable is set to the most recent tag (v2025.1.8).

No further action is required.

[Zeitsperre]

This is just waiting on Ouranosinc/xclim-testdata#30

[Zeitsperre]

One thing I'm wondering about are the names of these indices/indicators. Given that they can accept an indexer, would it make more sense to general the indices more and rename the indicators to use "Christmas" as well as hardcode them to December 25th?

@tlogan2000 any opinions on this?

[aulemahal]

If you hardcode the date to a specific date, then there's no need for a compute function, you could simple use generic.count_occurrences. And I would argue that the more general version of holidays_snow_days is snd_days_above.

I think generalizing holidays_snow_and_snowfall_days would best be done by implementing a count_multivariate_occurrences.