General refactoring to address issue #16

General refactoring to address outstanding issues with hierarchy importing detailed in issue #16:

 - Cleanup hierarchy importer test and add (currently failing) asserts for child IDs
 - Simplify PermissionScope and the SystemScope singleton to the bare bones
 - Add a method to PermissionScope that fetches its id chain as a string,
   and remove the method from IdGenerator that takes a PermissionScope
   in favour of this path of strings
 - Importers now use a stack of parent ids to define the depth. The mechanism
   to retrieve these IDs is still a total mess. Fixed several tests
   that had arbitrary and incorrect asserts.
 - Make Bundesarchiv, Niod methods that fetch identifiers less gnarly
 - Add lots of assertions to hierarchy importers to check ID generation
   and sanity check the resultant node hierarchy
 - Add FIXME concerning extreme dodginess