Matt ayman frames

Redo of the merge of Frame, RigidFrame, FixedFrame, Station and Marker ModelComponents along with code to convert Marker to new format for serialization/deserialization and inclusion of FrameSet in model.
@sherm1 @msdemers @aseth1 or @chrisdembia Please review and let us know your feedback.
If this method actually re-measures the point location from a different frame as the comment suggests, then it should not be called "express". Simbody's API (and conventional usage) uses "express" to mean a change of basis *without* changing the physical quantity. That is, say you have point P's location r measured from frame A's origin. That is a physical quantity; the vector AP. To write it down you need an expressed in frame, typically it would be A. Now if you re-express in B, you have the same *physical* quantity r=AP, but you have written it down differently. That is distinct from determining the location in B, which is a different physical quantity BP.

Simbody's `MobilizedBody` API as a large family of routines for performing these operations, with the verb "express" limited to operations that don't change the physical quantity. I think OpenSim's Frame API should follow that convention as much as possible to avoid confusion.


My original intention in Simbody's API was to use "location" exclusively to mean "location in space", i.e. "position with respect to the Ground frame". But I think it got away from me and ended up being synonymous with "position".