BUG: Fix import of hierarchy node with existing ID

Note that it does not fix the import of annotation hierarchies.
For some reasons (see vtkSlicerAnnotationModuleLogic::OnSceneNodeAdded),
the hierarchies are automatically loaded into the wrong hierarchy node
parent.
Issue #2849

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21561 3bd1e089-480b-0410-8dfb-8563597acbee