Fix vtkHyperOctree copy functions.

DeepCopy used to perform a ShallowCopy instead.
A new DeepCopy operator has been added to the internal structure
in order to copy the tree correctly.

Change-Id: I393b4447b2f1014604e9fdbd6e7286f8ec563701
Merge topic '13819_fix_hyperoctree_copy' into master

743c380b Fix vtkHyperOctree copy functions.