Add shape vs list-of-shapes collision method

and list-of-shapes vs list-of-shapes
add way to enable Model for collisions
add API stubs to RagDoll for SimulationEngine stepForward
enable shape collisions for MyAvatar skeleton

also some stubbery for main simulation loop
Shape::get/setCenter() instead of get/setPosition()
Shape uses "center" instead of "position"
SphereShape, CapsuleShape support derived classes
add VirletSphereShape and VirletCapsuleShape