Combining attrs of member DataArrays of Datasets

[johnomotani]

While looking at #4009, I noticed that the combine_attrs kwarg to concat or merge only affects the global attrs of the Dataset, not the attrs of DataArray or Variable members of the Datasets being combined. I think this is a bug.

So far this PR adds tests that reproduce the issue in #4009, and the issue described above. Fixing should be fairly simple: for #4009 pass combine_attrs="drop" to combine_by_coords and combine_nested in open_mfdataset; for this issue insert the combine_attrs handling in an appropriate place - possibly in merge.unique_variable. I'll update with fixes when I get a chance.

• Closes Incoherencies between docs in open_mfdataset and combine_by_coords and its behaviour. #4009
• Tests added
• Passes isort -rc . && black . && mypy . && flake8
• Fully documented, including whats-new.rst for all changes and api.rst for new API

[johnomotani]

@TomNicholas do you think we should make this test pass (i.e. support combining attrs on DataArrays contained in the Dataset being loaded)?

Proposal for a fix:

• Support open_mfdataset-style attribute combining by upgrading the combine_attrs argument to combine_nested/combine_by_coords (and probably the other functions that have a combine_attrs argument) so it can be passed one of the Datasets or DataArrays being combined/merged, and will then take the attrs from that.
• In open_mfdataset use the attrs_file argument to get the Dataset to take the attrs from, and pass that to the combine_attrs argument of combine_nested/combine_by_coords.