Add missing_dims argument allowing isel() to ignore missing dimensions

[johnomotani]

Note: only added to DataArray.isel() and Variable.isel(). A Dataset should include all dimensions, so presumably it should always be an error to pass a non-existent dimension when slicing a Dataset?

• Closes Allow isel to ignore missing dimensions? #3866
• Tests added
• Passes isort -rc . && black . && mypy . && flake8
• Fully documented, including whats-new.rst for all changes and api.rst for new API

[max-sixty]

Thanks @johnomotani ; appreciate the PR.

I'm not strongly opposed to this. I haven't personally had an instance where I've wanted this keyword, but that's not overly representative of others' cases.
What do people think, @pydata/xarray ?

Separately, if we do merge, I think we should align with the keywords on drop_vars: raise / ignore

[dcherian]

I don't see why Dataset.isel is different.

+1 for using the drop_vars kwarg

[johnomotani]

The Linux-py38-upstream-dev test is failing in cftime tests at various places similar to this

        with create_tmp_file() as tmp_file:
            original.to_netcdf(tmp_file)
            with pytest.warns(None) as record:
                with open_dataset(tmp_file, use_cftime=False) as ds:
                    assert_identical(expected_x, ds.x)
                    assert_identical(expected_time, ds.time)
>               assert not record
E               assert not WarningsChecker(record=True)

Don't think it's related to this PR...

[keewis]

you're right, that's #3928

[max-sixty]

Great, thanks a lot @johnomotani !