Added some commnets to clarify the role of some of the relationship types
Fleshing out Group Node, ID Node + A mid-level "data" node

* The "data" nodes are mid-level, and intended to sit between ID/Group
  "outer" nodes (i.e. optimisation and grouping nodes) and "inner nodes"
  to make it easier to query for stuff like bone-deps. They are only used
  since we'd be creating them in the meantime anyways.

  Then again, these may still go away later, if this just makes things too
  hard...

* TODO: some avenue for "dependency injection" within groups is needed
  for things like IK Trees being built with different sizes...