ENH: add the ability to import multi level model hierarchies from CLIs

Added to support passing a template scene to the Model Maker
that contains a multi level model hierarchy.

TODO: decide the best practice when a hierarchy is imported twice (for
example, running the model maker on the same label map more than once)
right now this will make a new hierarchy rather than over writing
the old one.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21621 3bd1e089-480b-0410-8dfb-8563597acbee