Also :+1: on @astrofrog's suggestion.  But this begs the question should we remove `transform_to` for Coordinate classes?
```
c = ICRS(...)
f = FK5(c)
```
Note that this does something potentially good because it provides a natural opportunity to provide additional metadata needed for a coordinate system transformation.  There has always been this problem that a generic `transform_to` method doesn't have specific knowledge of the additional metadata (e.g. obs_time) required for the transform.  But if using a class initializer like above is done then that knowledge is obviously there.

Caveat - I'm late for lunch and haven't thought this through carefully.  :smile: