Implement Leg and Activity Engines and prepare DRT functionality

[paulheinr]

This PR includes a couple of changes.

General architectural changes:

• introduce a Scenario struct that holds all the entities relevant for simulation (similar to the Java reference implementation)
• introduce ActivityEngine and LegEngine for separation of concerns
  • ActivityEngine holds agents, that perform an activity and releases them at the end
  • LegEngine performs legs of the agents on the network as well as teleportation. It also communicates updates to other partitions.
  • InternalInterface is used to transfer agents between both engines (as the reference implementation)
• introduce the possibility to carry passengers in vehicles
• introduce vehicles

DRT/Service architecture related changes:

• define DRT event structs
• add (unused and untested) messaging methods for service calls
• add simple DRT test scenario