Coordinates ape5 sky generictrans (probably not for merging)

This now conflicts with your branch, @taldcroft, but you can just look at this one commit to see the idea.  A natural extension that might make it all easier is to implement the attributes inside `SkyCoord` at the *beginning* as `GenericFrame`.  It still might need to be updated if there's overlapping `new_frame` attributes, but that's the idea, anyway.