Part combiner: separate split state and voice names

[Dan Eble]

To those familiar with the part combiner:

I would like to break the rigid link between the split states (apart, 
unisilence, etc.) and the names of the voices into which the 
Part_combine_iterator routes events.  I would like the Scheme portion of the 
part combiner to dictate the names of the voices to use at every moment.  This 
probably needs to be an addition to the split state name, not a replacement of 
it, but I’m not sure of that.  I expect the Part_combine_iterator to become 
much simpler.

I hope that the ability to route events in new ways without having to name each 
combination will simplify fixing some bugs and adding features.

If you would encourage or discourage me from this, please speak up.  I expect 
to experiment with it as I find time.

Regards,
— 
Dan