Major refactoring:

  - The score is read from file and contains "annotations"
    which can manage paramters like pitch range, probabilities
    of double notes, etc., in addition to the cues for the parts.

  - The score is played by a new class, "Ensemble.pde"

  - The old "NoteSequence" class is now "Generator".

  - There is still work to do, e.g., the bass begins, but I don't
    hear it until the tenor enters.