BulletSim: clean up TargetVelocity implementation by using the default defn in the PhysicsActor base class.
BulletSim: make sure vehicle physical properties are set when going
physical by delaying setting until pre-step time. Change vehicle.Refresh()
to schedule the pre-step setting.
Comments and updating of TODO list.
BulletSim: fix crash caused when linksets were rebuilt. A problem added
when individual child pos/rot changes were implementated a week or so ago.
Remove some passing of inTaintTime flag when it was never false.