Add Interval class. Incorporate note group properties into Note class, but instead of containing notes, it contains intervals.
Fix rendering pipeline (conversion of part into note sequences and then instructions).
Remove NoteGroup class.
Comment unused Performer methods.
Rewrite instrument architecture based on keys. The Instrument class provides the logic for adding, removing, and using keys to render notes. THe dervied instrument class provides a means to adjust parameters and handling the actual rendering for each key.