Add ordinal value to r_node

[DougLau]

Each r_node has a single ordinal value determining its position in the road network.

• Valid station, entrance and exit r_nodes are given ordinal values
• A gap in assigned ordinal values indicates the start of a new corridor
• If the ordinal value is NULL, it is not connected to the network.

After one or more r_nodes are moved/updated, the entire network must be recalculated.