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.
@msdemers my only dispute is with RigidFrame containing the Transform. The problem with this picture is that every FixedFrame and Body now has this _mbFrame sitting around. Are  those values meaningful? It is identity for a Body? It seems that only FixedFrame needs to hold this information.  What is it for some other derived RigidFrame, like RigidNode or MasslessBody? It seems to muddy the picture to me. I can see that you need a place to hold the complete transform from a chain of FixedFrames, but I am not sure it belongs in RigidFrame.

If a RigidFrame is mobilized body and a fixed transform can you please explain to me what a FixedFrame is?

I am only advocating for a slightly stricter definition that RigidFrame is the fundamental rigid element in OpenSim and thus the incarnation of a MobilizedBody, the same way Body used to be and how OpenSim::Force and Constraint are direct counterparts of similar SimTK elements. Then FixedFrame has a clear meaning and purpose. I don't see how that breaks what you are describing.