Fix assert message
Add Q_ASSERT to forbid tuplets with <= 1 element
More rough off-time quantization of human-performed pieces
Simplify code: notes and chords are never erased
Fix tuplet iterator after chord merge + add Q_ASSERT
Fix the case when chord interval has zero length
Set tuplet voice when create notation
Fix useless tuplet deletion during voice separation
Add constness to some containers and iterators in quantization
Fix quantization of chords from previous ranges