ENH: Update parse local data to follow changes in subject hierarchy

Parse local data ('Create hierarchy from local directory structure' context menu action for scene in subject hierarchy) now creates folder nodes instead of subject and study, because we cannot guarantee that the number of directory levels accommodates this condition.
Removed the concept of ChildLevelMap and related code, due to fewer levels (now we only have Subject, Study and Folder, there is no separate Patient and Generic series) and the general nature of Folder (there is no need now to create a new type of hierarchy node for every grouping concept).

Fixes #3892

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23795 3bd1e089-480b-0410-8dfb-8563597acbee