Rename coordinate classes, removing `Coordinates`

In preparation for the 0.3 release (and the intended larger changes in coordinates in the *next* release), this PR changes the names of all of the coordinate classes to remove the "Coordinates" from the end.  E.g., it changes `ICRSCoordinates` to `ICRS`.  

While initially it seemed like the "coordinates" part would make more readable code, I've found in practice that it's always clear from context that these things are coordinates, and the names are generally unique enough, so the suffix is just annoying.

This has a backwards compatibility layer that continues to allow the old names to be used, but gives a warning and yields the new class.  The one downside is that that means they have to be included in the class inheritance diagram, because they are actual classes. Presumably we'll just remove that in the next release, though, so I don't consider it too big of a deal.