Pass on vehicle to problem in a cleaner way

[timmercy]

Trailer example:
For now you make a vehicle, make a trailer, add the vehicle as lead_veh to the trailer and pass the trailer on to problem. This means that you will only simulate the trailer. But the problem should know the trailer AND the vehicle. Therefore I manually add the vehicle to optifather (and not to problem) by putting in p2p_trailer.py:
problem.father.add(vehicle)

Maybe we should think of a cleaner way?