BulletSim: freshen up the code for constraint based linksets.
Add experimental stubs for an extension function interface on both
PhysicsScene and PhysicsActor.
BulletSim: move collision processing for linksets from BSPrimLinkable
into the linkset implementation classes.
Add HasSomeCollision attribute that remembers of any component of
   a linkset has a collision.
Update vehicle code (BSDynamic) to use the HasSomeCollision in place of
   IsColliding to make constraint based linksets properly notice the ground.
Add linkset functions to change physical attributes of all the members
   of a linkset.
BulletSim: only create vehicle prim actor when vehicles are enabled.
BulletSim: change BSDynamics to expect to be passed a BSPrimLinkable
and start changing the logic to handle the base prim as a complex
object (ie, a linkset).
BulletSim: Add logic to linksets to change physical properties for
whole linkset.
Override physical property setting for BSLinksetCompound as there are
not children to the compound spape.