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?
Maybe make it just a bit clearer that this is actually the roll angle *off* of the Galactic?  E.g., something like  "The angle to rotate about the final x-axis, relative to the orientation for Galactic.  I.e., if this is 0, the final x-z plane will align with the Galactic coordinates x-z plane.  Unless you really know what this means, you probably should not change this!"