Support pandas.MultIndex axes on xray objects

[shoyer]

• Appropriate casting with xray.Coordinate
• Call out to MultIndex.get_locs in indexing.convert_label_indexer
• Get multi-index support working with .loc and .sel()
• Serialization to NetCDF
• Consider stack/unstack and pivot like methods

Not all of these would be necessary for an MVP.

Right now we don't consider the possibility of a MultiIndex at all -- at the very least it would be nice to give an error message.