ENH: Removed explicit level member from subject hierarchy item

The reason for this is that level was somewhat confusing: all items for data nodes had series level by default, but it was not explicitly specified what it caused, and also the role of subseries level was unclear. Moreover as level was a member, it was expected to be non-empty, however in some cases it was not useful
Level is still available and used, but it's now a subject hierarchy item attribute, which is not mandatory. Convenience functions were preserved for level getting, setting, and checking, and added for creating patient, study, and folder items. So level argument was removed from functions where it was not useful, and the code is now a bit simpler

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25896 3bd1e089-480b-0410-8dfb-8563597acbee