[WIP] added protect_dataset_variables_inplace to open_zarr

[rabernat]

This adds the same call to _protect_dataset_variables_inplace to open_zarr which we find in open_dataset. It wraps the arrays with indexing.MemoryCachedArray.

As far as I can tell, it does not work, in the sense that nothing is cached.

• One possible way to close expose zarr caching from xarray #2812
• Tests added
• Fully documented, including whats-new.rst for all changes and api.rst for new API

[kmuehlbauer]

@rabernat Given the age of this, is this still in the works?

[rabernat]

Same answer as on #2814. Not quite sure whether there is interest in this feature.

[kmuehlbauer]

Thanks @rabernat for the quick reply. I was skimming old issues/pr to see if there are some which might be closed.