Fix coordinate transforms to old names

This fixes two things about `coordinates`:

1. #1614 renamed the coordinates classes but kept shell classes that allowed the old names to be used.  This accidentally had the side effect of making it impossible to use `transform_to` to get to a class using the old naming scheme.  That defeats the point of bothering to allow the old names in the first place.  There's a possibility of some confusing because the result is actually something with the *new* name for the class, but that was already true even when instantiating using the old name.
2. It takes care of a subtle bug if there exists any system that can be transformed *to*, but not *from*.  We didn't have any before, but the solution to # 1 introduces some, so this fix is necessarily packaged together.

These are both definitely bugfixes and fairly straightforward, so if the tests pass, it should be mergeable (and certainly by Friday's RC).