divisi: add code by David Kastrup
Initial version of divisi/partcombine framework
ensure synchronization when input lengths don't match
use \removeWithTag instead of \keepWithTag.

That method will at least preserve music that the user has tagged
for some other purpose, and it co-exists with \removeWithTag for other
purposes.

Just in case anyone uses the \divisibleStaff with his own tags, and might
not think to include 'together in \keepWithTag score \divisibleStaff ... ,
I added a function to clean up the divI/divII/together tags when we are
done with them.