more work on MatterSubsystem interface, with some doxygen comments now
more work on MatterSubsystem interface, with some doxygen comments now

Gjacquenot/simbody,robojukie/simbody,elen4/simbody,scamille/simbody,simbody/simbody,robojukie/simbody,scamille/simbody,chrisdembia/simbody,elen4/simbody,elkingtonmcb/simbody,elkingtonmcb/simbody,robojukie/simbody,elkingtonmcb/simbody,chrisdembia/simbody,Gjacquenot/simbody,chrisdembia/simbody,simbody/simbody,scamille/simbody,Gjacquenot/simbody