WWWGraph - A class encapsulating the WWW graph model
This class encapsulates the WWW graph. The WWW graph actually has several `helper' graphs that implement the cost model.
- a graph for downward edges
- a graph for side edges
- a table of targets for side edges that come from outside
- a reachability table
We ignore back edges.
Finished, save the the graph objects.
Cache the graph object references.
Add an edge to the WWW graph from one URL to another URL. This function determines which subgraph to add it to internally.
Create the reachable graph from the side and downward graphs.