Provide more support for xr.Datasets as inputs

[nicholasloveday]

Some metrics only support xr.DataArrays. I have received feedback from users who would like to be able to use scores primarily with xr.Datasets.

[nicholasloveday]

For functions that currently take an xr.DataArray and return an xr.Dataset with under/over forecast penalties, consider returning an xr.DataArray with a new dimension called "components".

If we go down this path, we will have to update Jive to be compatible if it expects an xr.Dataset.