Update open_dataset backend to ensure compatibility with new explicit index model

[lukasbindreiter]

Based on feedback on this issue: #7139

• Closes xarray.open_dataset has issues if the dataset returned by the backend contains a multiindex #7139
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[benbovy]

Thanks @lukasbindreiter for this PR!

I think it works in most cases but maybe not in all cases. CopyOnWriteArray is not useful for an (immutable) indexed coordinate variable, but wrapping variable data in a MemoryCachedArray might still be relevant for custom "lazy" indexes. Unfortunately, we don't have any example yet for the latter. Let's go ahead with your fix and address the general case later.

Would it be possible to add a test with a multi-index? Not sure how best to do that as I'm not familiar with this part of Xarray (backends) and how tests are written there.

[lukasbindreiter]

Would it be possible to add a test with a multi-index?

I added such a test now (which fails with the old version of _protect_dataset_variables_inplace but runs through with the updated version of this PR)

[benbovy]

Just one small suggestion below.

Could you also update whats_new.rst with your contribution please?

[lukasbindreiter]

Could you also update whats_new.rst with your contribution please?

Done 👍

[benbovy]

Thank you @lukasbindreiter! Merging.

I notice that this is your first contribution to Xarray, welcome!