Implement galactocentric frame

This is a first stab at implementing a frame for galactocentric coordinates. Some open questions in my mind: 

* Do we want to allow a transformation to a left-handed system as well? (because some people are crazy)
* Can we think of better names for the frame attributes?
These can all be `QuantityFrameAttribute`s once #3217 is in.  can you add a TODO about that now?  Hopefully we'll remember anyway, but better safe than sorry...