Add method to unjoin OBBTrees for individual volume sets

[Paul Wilson]

Supports #77.

This is to allow for rebuilding those OBBTrees after some surfaces have moved

[Paul Wilson]

Algorithmically, this results in the deletion of the portion of a volume OBBTree that results from joining surface trees, without deleting the surface trees themselves.

[Paul Wilson]

Psuedo-code

unjoin(vol_root)

del_list.push(vol_root)
child_list.push(get_children(vol_root))
while child_list not empty:
     child = child_list.pop()
     if child not surf_root:
          del_list.push(child)
          child_list.push(get_children(child))
for node in del_list:
     delete(node)

[Paul Wilson]

addressed by pull request #391