Option to force all vehicles to service locations

We should provide an option to force all vehicles on the road.
Probably goes hand in hand with the fairness constraint, see https://github.com/mapbox/node-or-tools/issues/1.

---

Implementation: remove the depot from the start's `NextVar`, so vehicles _have to_ service locations:

```c++
model.CloseModel()

for (auto vehicle = 0; vehicle < numVehicles; ++vehicle) {
IntVar* const start = routing.NextVar(routing.Start(vehicle));
for (auto node = routing.Size(); node < routing.Size() + routing.vehicles(); ++node)
start->RemoveValue(node);
}
```

Start by reading the linked fairness-constraint issue and locating the vehicle-routing setup in the bindings; the issue provides the proposed NextVar constraint. Check how an option is exposed, then verify with a routing example that every vehicle is required to service a location.

Written by the indexing model from the issue text.