Reach better solutions using unassigned jobs

[jcoupey]

The job removal/re-insertion strategy applied after each local search step was primarily designed to improve the way already assigned jobs where handled across multiple routes. It can also help improve the solution with regard to unassigned jobs, but fails to do so in some situations where obvious better solutions are close (#319).

We should introduce a new local search operator that goes through each step for each route to evaluate the gain of removing it and inserting another unassigned job somewhere in the route. This would differ from other existing operators in that it might change the set of (un)assigned jobs and thus potentially change priority on top of travel time cost.

[jcoupey]

Landed with #383.