Systematically identify coordinate types for xarray #860

This means identifying that time1 is the proper time coordinate/variable, that lat or y is for the y-axis, etc. This would be an addition to the functionality under the parse_cf() accessor.

My first inclination is a dictionary mapping names, or providing a mapping to the coordinate variable. The latter sounds less verbose. Another option is to just flat out rename the coordinate, but I'm not sure how messy that will be.