Avatar interaction first pass

This is preliminary progress: an avatar can reach out with a hand and bump another's arm, causing it to twitch.  The twitch is not realistic, and if the hand bumps the other's hand then the fingers can get mangled into unatural positions.  Also, the collision is only between your avatar's hand joint (not the entire hand -- no fingers) which collides approximately like a sphere and the other avatar's arm.   Arm-vs-arm doesn't work nor can we yet grab the avatar's torso.

There are two main problems going forward:

(1) Some bones of the avatar skeleton are not yet free to move.  In particular the torso and shoulders.

(2) We don't have a relaxation model for the avatar's skeleton, so bones that are free to move can be put in many awkward positions and they will stay there.  At the moment this only includes the fingers and wrists.  The hands are an exception -- when not driven by an external signal the hand positions are specified and as a result the arms are pulled into into realistic orientations.