Added interval twCRPS for ensembles

[nicholasloveday]

I have added a convenience function for calculating the twCRPS for an interval. This pull request just shows works with a single interval. The future twCRPS tutorial notebook will show how to work with multiple intervals.

Development for new xarray-based metrics

• Works with n-dimensional data and includes reduce_dims, preserve_dims, and weights args.
• Typehints added
• Docstrings complete and followed Napoleon (google) style
• Reference to paper/webpage is in docstring
• Add error handling
• Imported into the API

Testing of new xarray-based metrics

• 100% unit test coverage
• Test that metric is compatible with dask.
• Test that metrics work with inputs that contain NaNs
• Test that broadcasting with xarray works
• Test both reduce and preserve dims arguments work
• Test that errors are raised as expected
• Test that it works with both xr.Dataarrays and xr.Datasets

Documentation

• Add the score to the API documentation
• Add the score to the included list of metrics and tools

[Steph-Chong]

Really minor feedback re. adding page numbers to reference. That's very handy an open access article just came out!

[rob-taggart]

Hi @nicholasloveday , looks good. Just a few comments about the docstring. General comment: I think we need to keep working hard to distinguish between threshold weights and weights applied in a weighted mean of scores

cc @tennlee , so you know I've done the review

[nicholasloveday]

Thanks for the feedback @Steph-Chong and @rob-taggart. I have updated the branch with your suggestions.