Make Marker into ModelComponent that derives from Station and modify tools accordingly.
Main changes in Marker:
1. Marker implementing ModelComponent interface.
2. bodyName property changed into Connector to a RigidFrame (in OpenSim::Station)
3. fixed flag has been removed (assumed false, tools now work but will be updated accordingly)

To support Marker functionality, RigidFrame has been amended with method getBody() to return ref. to the Body that the RigidFrame is attached to.
Replace unnecessary const_cast with dynamic_cast (since SafeDowncast doesn't handle const pointers)
Couple more comments.
Merge branch 'matt_ayman_frames' of https://github.com/opensim-org/opensim-core into matt_ayman_frames