keep a pointer to owning agent in AgentData
maintain a pointer to owning avatar from HeadData
pass the owning agent for VoxelAgentData constructor
use a NULL default for owningAvatar to simplify constructors
Merge remote-tracking branch 'upstream/master' into owner-pointers
Merge pull request #421 from birarda/owner-pointers

add pointers back to owning avatar and owning agent