Collision Shapes

Adding some Shape classes for a more featureful collision system.  Right now there are three real shapes and one base class:

Shape (base class)
SphereShape
CapsuleShape
ListShape

SphereShape and CapsuleShape have some unit test coverage in .../tests/physics/src/ but ListShape is not yet to be tested.

I've started using sphere and capsules for Model collisions but this is still a work in progress.  In particular colliding hands and avatars does not currently work.  Also, the transform offsets for the skeleton shapes appear to have some slight errors.  Dunno if these are a numerical error problem or incorrect logic, but I'm punting that problem for now and will revisit later.