Before the nutempo changes, a call to Region::set_length() only set the length
of the region. Now that Region::_length is a duple of (duration,position),
maintain thesame semantics so that the call only affects the duration part of
the duple. Theoretically we might at some point want/need
Region::set_length_and_position() but that is not the case at this point,
97e4a4f04a