Coordinates ape5 1to1 framenames

This PR addresses an issue that @taldcroft brought up in #28 : There's no really good reason for the feature that `TransformGraph` objects can have many names for each frame class.   In fact, that's probably more confusing than its worth.  So this changes things so that frames have a `name` class attribute that can be set explicitly, or if not, will be taken as ``lower()`` of the class name.  The functionality for adding names to `TransformGraph` is then removed in favor of using the names on the objects for the `TransformGraph` methods that deal with names.  If it does turn out in the future that aliases really are important in some context, they can always be added back in.

The presence of `name` attribute and the one-to-one assumption it allows then leads to various simplifications in `SkyCoord`... So you'll note this is @taldcroft's favorite kind of PR, with more code removed than added :wink:

Look good to you, @taldcroft ?