Fully support xr.Dataset input

[jbusecke]

Quick reminder here:

I guess an additional test (or a parametrization for the existing ones) would have been ideal. In case you are up for adding that, I would wait, but not a big deal in my view.

I agree, but that would take considerably longer and I'm going to take some time off starting today. I thought it was better to get the bug fix out as soon as possible.

I can help you out in the future if you want to add tests for datasets and make that aspect of xmovies fully supported.

Originally posted by @tomchor in #83 (comment)