Enhanced RouteSplit operator

[jcoupey]

The logic for the RouteSplit operator tries to split a single route into two routes for cheaper vehicles. This happens without any re-ordering, i.e. we look for a splitting rank candidate where the route cuts into two other valid routes. This works in many cases but is quite constrained since it:

• does not allow any sub-route reordering;
• does not allow fancy multiple splittings;
• does not allow splitting to more than 2 vehicles.

Now that we have #837, we could replace this approach by applying heuristics partially to the current route we look at with other empty vehicles.