rewrote and refactored MusicXML score parsing

more simple, more correct
clean up, clean up, everybody clean up

clean up some comments and some code. no semantic changes
Prune finger configurations

Don't even add fingering configurations that are implausible to the A*
search graph. We already know they're going to have a high weight and
won't be selected.
99% working

still a bug where, seemingly randomly, some note ids are removed from
the note and set to None. I have no clue where this is happening. Need
to debug. Will work on this tomorrow. A* is working though!!!