add_manager
Description:
Adds a sub manager to this.
Use this method to create the skeleton structure of a Tree. Note that a single TreeManager can be used by several Tree objects or several times in the same Tree's structure.
Please note that it's possible for mgr and sub to be the same object, but beware of the possible infinite recursive
behaviour in this case when creating children nodes (depending on the actual implementation of the
TreeManager).
Parameters:
| this |
a TreeManager object |
| sub |
a TreeManager object to add |