The various conditionals in Evoral::RangeList::subtract() appear to have
been there to work around
(a) coverage() not always returning the correct value, &
(b) the test suite assuming that the ->to point lies outside the range
Now that these are both fixed, the implementation of subtract() becomes
quite a bit clearer. I replaced the if()s with assert()s for now, but these
shouldn't trip if coverage() is working as I expect.
Also (attempt to) clarify the comments in subtract.
dd5fc9432f