Allow vehicles to carry multiple passengers

[paulheinr]

Right now, vehicles can only carry one agent (src/simulation/wire_types/messages.proto:34). For DRT, we need multiple agents in one vehicle.

Think about

• whether drivers get an extra role or not
• what happens if a vehicle gets parked (right now it returns the agent)

[paulheinr]

In classic MATSim, there is a distinction between Drivers and Passengers (org.matsim.core.mobsim.qsim.qnetsimengine.QVehicleImpl). There is a special Role org.matsim.core.mobsim.framework.DriverAgent, which equips the driver with perception of move over node and with the decisions to choose next link and to arrive.

Placing the agents on the respective seats (Driver/Passenger) is done by org.matsim.core.mobsim.qsim.interfaces.DepartureHandlers. There is one for each mode. For DRT, passengers are placed on the passenger seats, for car, they are placed as driver.