for when pointsets all share the same Z coordinate.
ENH: Move octree node bounds out of the octree_node structure and
     into vtkLabelHierarchy::Implementation::LabelSet where it
     belongs, as it is only there to accelerate the iterator
     BoxNode member.
ENH: Make octree nodes point to their direct parent and not the
     tree top so it is possible to keep track of things like
     the number of children below a level dynamically.
ENH: Adding a depth-first 3-D octree iterator that mirrors the
     2-D quadtree version for testing and comparison to FULL_SORT.
     The DEPTH_FIRST enum can be used to specify it.