Add a walking layer different from the transit layer

The Dijkstra algorithm cannot be easily adapted to take into account the estimated waiting times during shortest paths discovery while keeping the structure of the multi layer graph as it is now and maintain optimality (check in the documentation for an explicit example of that). It would require to change the structure of the multi layer graph by defining a walking layer different from the transit layer. Then, the waiting times could be worn by the transit links and a user who only walk would stay in the walking layer, without passing through a node belonging to another layer. On the one hand, it may result in a higher size of the graph. On the other hand, it would enable a higher flexibility in the costs definition.