BulletSim: non-functional updates. Comments and formatting.
Update TODO list.
BulletSim: comments and non-functional changes working toward the center-of-gravity implementation.
BulletSim: More tweaking on center-of-mass. Almost there. Changes have no effect if LinksetOffsetCenterOfMass=false (the default).
BulletSim: implementation of linkset center-of-mass.
Default off, for the moment, until more testing.
Add separate thread and center-of-mass flags to OpenSimDefaults.ini.
Clean up comments in OpenSimDefaults.ini.