Priority-improving move selection only accounts for priority

[jcoupey]

When picking the best option in local search, we first lookup moves that improve priority score:

vroom/src/algorithms/local_search/local_search.cpp

Lines 1815 to 1822 in 405ccd1

	for (unsigned s_v = 0; s_v < _nb_vehicles; ++s_v) {
	if (best_priorities[s_v] > best_priority) {
	best_priority = best_priorities[s_v];
	best_gain = best_gains[s_v][s_v];
	best_source = s_v;
	best_target = s_v;
	}
	}

The problem here is that there is no account for gain (or solution deterioration), so if multiple moves yield the same priority score gain, the first one will be picked, possibly shadowing another that is better in term of cost.

Also we used to only have UnassignedExchange being able to change the priority score, leaving the number of assigned tasks identical. Now PriorityReplace is able to both improve priority score while deteriorating the number of assigned tasks. We should also be able to pick the least task removal for equal priority score improvements.

[jcoupey]

Fixed along with the changes in #1210