Mod block

There are lot of things in this PR : 
 * First there is a refactorization of read_path: it's now done in a function accepting a visitor.
 * Then we remove the journey_pattern_point_connections
 * Finally we introduce a next_vj and prev_vj in vehicle_journeys, and we use this in the algorithm.

In the algorithm, at the end of a computation of a journey_pattern if the last taken vehicle_journey has a next_vj, we apply it.