Blocking centroid flows option

[djfrancesco]

if blocking_centroid_flows=True when creating the graph, two different kind of od nodes are created: origin and destination, otherwise only od nodes are created. In the former case, egress connector edges point toward destination nodes, while access connector edges start from origin nodes. All these nodes are located at the zone centroids. The origin and destination are not directly connected, which is how the centroid flow is blocked.