Created "indexers" Subdirectory; Updated Importing

- all imports from vis should now start with "vis", which may take some
  tricky maneuvering to ensure constant working
- IntervalIndexer, NoteRestIndexer, TemplateIndexer, and all new Indexers
  are now in their own module in vis.analyzers.indexers
- revised indexed_piece.py and run_tests.py for new importing
Simplified Indexer & IndexedPiece Tests

- in TestIndexerThreeParts, simplified set-up
- in TestNoteRestIndexer, bwv77_soprano and bwv77_bass are static class
  variables, because...
- in IntervalIndexer, we use bwv77_soprano and bwv77_bass from TNRI, so
  they are no longer generated at run-time by a NoteRestIndexer instance
- in IntervalIndexer, no tests require a NoteRestIndexer
- two IndexedPiece test classes renamed; one added to run_tests.py