NaN-handling for ensemble CRPS

[frazane]

This pull request introduces handling of NaN values in ensemble CRPS scoring functions via NaN policy selection. Three policies are supported:

• 'propagate': if any member of an ensemble is NaN, the function returns NaN
• 'omit': the function will omit NaN values from the computation
• 'raise': the function will raise an error if any NaNs are present in the input arrays

These are now values given to a new nan_policy parameter for crps_ensemble, twcrps_ensemble, owcrps_ensemble, and vrcrps_ensemble.

[frazane]

@sallen12 I reimplemented the NaN-handling like you suggested, using the ensemble weighting functionality and setting NaN members weights (and values) to zero. I only realized this was indeed an option after merging main into this branch. My bad!

This is now greatly simplified. There is still a couple of edge cases that need to be handled (AKR estimators). Perhaps we can have a chat about this soon.

[frazane]

@nicholasloveday any feedback here would be welcome!

[frazane]

Note: I left the ad-hoc AKR estimators for the 'omit' nan policy, but we are not using them in the high level API. We will decide later. For now, choosing AKR estimators or INT with 'omit' policy will raise a NotImplementedException.