Coach shuttling example: add shadow variable BusStop.passengerCountInBus

(naming not fixed, suggest better names)

First take a look at:

• docs "Shadow variableses" section
• ArrivalVariableListener

Idea:
Each BusStop know how many passenger are in the getBus() upon departure from this busstop. This is the sum of:

• the passengerQuantity at this stop
• the passengerCountInBus in the bus of the previous stop of this bus
• the passengerCountInBus for every shuttle transferring here at this stop of the previous stop of that shuttle

Adjust DRL and ConstraintProviders to take advantage from this.

Reqs:

• Verify that after a 1000 LS steps, it has the same score with DRL before and after the model changes.
• Report score calculation count difference between DRL before, DRL after and CS (as a 1-line comment on this jira is fine)