Enhancement request -- make TabStaff work properly with non-monotonic stringTunings

[Carl D. Sorensen]

Right now, the TabStaff will only automatically calculate string and fret
numbers properly for stringTunings that are monotonic in pitches (i.e.
strings consistently increase in pitch).

Some instruments, e.g. Ukelele and Venezuelan Cuatro, do not have this
characteristic, so strings must always be specified.

It would be nice (and I think it's doable) to adjust string-fret-finger to
work with non-monotonic StringTunings.

I'm posting this because I don't think this change should be made before
2.12, but I want to have something to tickle my memory.

Thanks,

Carl Sorensen