Support alternate tunings and MIDI constraints

[infojunkie]

The current conversion script does not fully support microtones nor alternate tunings:

• It produces pitch bends for any pitch that's altered outside of 12-TET
• It generates a MIDI note without sharps or flats for pitches above
• It does not support changing the tuning of a score with a user-specified tuning

Instead, we need a more comprehensive treatment of alternate tunings that takes into consideration MIDI constraints:

• Tuning(s) can be detected by linking MusicXML accidentals to their (implied) tuning
• A global tuning can be specified to override the implicit tuning(s) (like the globalGroove parameter)
• Pitch bends may not be desirable because they affect all notes within a channel
• MIDI Tuning Standard (MTS) exists to support alternate tunings but is not supported by all synths
• MIDI Polyphonic Epression (MPE) exists to support individual pitch changes but is not supported by all synths