Revert "Revert "BulletSim: freshen up the code for constraint based linksets.""
Found that the vehicle movement problem was not caused by these physics changes.

This reverts commit 44543ebe638f391fc1c7ff532fe4470006dec55a.
Revert "Revert "Add experimental stubs for an extension function interface on both""
Found that the vehicle movement problem was not caused by these physics changes.

This reverts commit 89857378ce79f93a265bc1eb151e17742032abfa.
Revert "Revert "BulletSim: move collision processing for linksets from BSPrimLinkable""
Found that the vehicle movement problem was not caused by these physics changes.

This reverts commit c45659863d8821a48a32e5b687c7b2a6d90b0300.
Revert "Revert "BulletSim: only create vehicle prim actor when vehicles are enabled.""
Found that the vehicle movement problem was not caused by these physics changes.

This reverts commit 5f7b2ea81b95a60e882bc65b663a2c9fe134f92a.
Revert "Revert "BulletSim: change BSDynamics to expect to be passed a BSPrimLinkable""
Found that the vehicle movement problem was not caused by these physics changes.

This reverts commit 7b187deb19517aa7c880458894643a5448566a94.
Revert "Revert "BulletSim: Add logic to linksets to change physical properties for""
Found that the vehicle movement problem was not caused by these physics changes.

This reverts commit 84d0699761b8da546f9faef084240d7b15f16321.
BulletSim: Turn on center-of-mass calculation by default.
Reduce object density by factor of 100 to bring physical mass computations
into a range better suited for Bullet.