implement map_over_datasets kwargs

[kmuehlbauer]

• Closes allow kwargs in map_over_datasets? #10009
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[TomNicholas]

This looks good! Though needs some docs, even just the docstring.

[mathause]

Thanks. One question on the implementation.

[kmuehlbauer]

Though needs some docs, even just the docstring.

Yes, please suggest or add as you see fit.

[mathause]

I guess "Allow kwargs in map_over_datasets" is enough for the changelog entry.

---

I'll approve. Just want to add, that the old implementation allowed the kwargs to be a DataTree, while this does not (I assume this is a deliberate decision in the re-implementation?). I don't want to hold up this PR for this reason but just remark that this makes using it ever so slightly more annoying, i.e. you may have to re-arrange your inputs to accommodate for it.

https://github.com/xarray-contrib/datatree/blob/5f3956ffe80e686dd3df54ee8cef9ff56c158e76/datatree/mapping.py#L162

[TomNicholas]

the old implementation allowed the kwargs to be a DataTree, while this does not (I assume this is a deliberate decision in the re-implementation?)

The implementation as of this PR matches apply_ufunc, which also doesn't allow you to pass xarray objects as kwargs.

[TomNicholas]

Should we show / state that the *args can be DataTrees while the kwargs cannot? This is shown clearly in the apply_ufunc docstring.

[kmuehlbauer]

@TomNicholas Is this now more explicit in the docstring?

[TomNicholas]

Thank you @kmuehlbauer - just one nit, but I'm approving.

[mathause]

Thanks!