BUG: Subject hierarchy nodes now created if importing old scene

If a scene was imported which was created before the implementation of automatic subject hierarchy node creation, then the SH nodes were not created. Many users use such scenes and they would consider this a bug. Now this is fixed.

Fixes #3902

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23796 3bd1e089-480b-0410-8dfb-8563597acbee