BulletSim: finish the post step event for physical object actions. Modify vehicle to use post step event for logging.
BulletSim: parameterize several vehicle debugging values: physical linear and angular force factors now default to less than 1 (0.2) vehicle friction and restitution now default to low values
BulletSim: first attempt at reporting top colliders
BulletSim: parameterize the value for gravity reduction for ground vehicles on the ground. Set defaults for vehicle factors to one. Debug logging changes.
BulletSim: fix compile error from last commit
BulletSim: disable center-of-mass computation because it does not work yet
BulletSim: add framework for BulletSim unit tests. No tests yet.
BulletSim: reinstate the supression of rotational velocity for vehicles
BulletSim: simplify the initialization of some of the parameters.
Disable vertical attraction for vehicles by default (for the moment).
Fix bug where vehicle would go crazy when velocity got above a
   certain speed.