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?
I only saw this sentence after my comment and understood the orientation ... but yes, maybe a bullet list explicitly stating where x, y, z are pointing would be better.