Make parts generation smarter using divisi and unis.

I the forums including https://musescore.org/en/node/310956 there have been discussions over the years to have smarter parts generation for instruments that change between divisi and unis./a2. Some things that should be considered:

a2 (or a3, a4) indicates all of the instruments play in unison

chords should be divided among the parts if there is no unis. (like on a violin) or if there is an explicit divisi

Voices should continue to work the way they currently do

The text identifying parts should allow customization.