working with transit modeling or different graph layers

[robfitzgerald]

in the case of transit modeling,

• we need edges appended to the graph which connect station stops
• we need a RoadClassFrontierModel to know the mapping from these edges to a "transit" class name
• we need a traversal model that knows how to find the transit edges in the graph by id

appending to the graph could take a few forms:

• the user needs to add the links to the edges-complete file before running
  • cumbersome
• TraversalModelBuilder::build can append links to the graph
  • all traversal models needs to keep track of the offset index + number of edges associated with the traversal model
  • how do we inform the RoadClassFrontierModel what the edge ids are?
• TraversalModelService has a register_edges method
  • similar to how we update the state model
  • in order to know what the index will become, we need to have the un-modified graph in scope to check it's .size

[robfitzgerald]

sketch of a solution...

1. a TraversalModel can create links, is called during CompassApp::try_from when creating a Graph
2. a Graph::extend or Graph::merge method which adds vertex + edge data to the graph
3. ~finally, a TraversalModel::register_graph_update() which takes the tuples and stores whatever information is relevant, which is called for every traversal model~