Fairness - balance number of locations serviced per vehicle

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We want to be able to balance the number of locations serviced by each vehicle to be as even as possible. The use-case is for example drivers who are getting paid (at least partially) by number of deliveries.

This constraint should be behind a flag and optional.

---

Implementation: use the underlying solver to
- Establish a vehicle vars <-> vehicle counts relationship via the `Distribute` constraint
- Add a `Deviation` constraint for vehicle counts targeting the `deviation` var
- Minimizing the `deviation` var

Roughly as follows:

```c++
std::vector vehicleCounts;

for (auto vehicle = 0; vehicle < numVehicles; ++vehicle)
vehicleCounts.push_back(solver->MakeIntVar(0, numNodes));

solver->AddConstraint(solver->MakeDistribute(vehicleVars, vehicleCounts));

auto* deviationVar = solver->MakeIntVar(0, numNodes * numNodes);

solver->AddConstraint(solver->MakeDeviation(vehicleCounts, deviationVar, numNodes));

model.AddVariableMinimizedByFinalizer(deviationVar);
```

Non sono indicati né un file né un test. Inizia individuando il modello di instradamento dei veicoli e la relativa configurazione dei vincoli opzionali, quindi esamina come il solver sottostante espone Distribute, Deviation e la minimizzazione tramite finalizer. Il lavoro è completato quando un flag opzionale abilita conteggi equilibrati delle posizioni per veicolo e il solver ne minimizza la deviazione.

Scritto dal modello di indicizzazione a partire dal testo della issue.