Unnecessary range copy in vrptw::PDShift

[jcoupey]

We're making a copy of the "remaining" source range in the vrptw::PDShift operator ctor:

vroom/src/problems/vrptw/operators/pd_shift.cpp

Lines 35 to 36 in 0aeb2cc

	_source_without_pd(s_route.begin() + _s_p_rank + 1,
	s_route.begin() + _s_d_rank),

This means a lot of overhead for a downstream use where we only use iterators in validity checks. The tricky part is that the copy is indeed necessary inapply since we can't replace part of a route while iterating over a subset of the same route. But we could make it so that the copy only happens in apply which is only called a marginal number of times across all instances of the operator.