The addUndirectedEdge method adds the given edge's source node-id and
destination node-id to the adjList twice: once with the source
node-id as the key and next with the desitnation node-id as the
key.
Derived from class Model, this abstract class serves as a parent
class for models that will ultimately generate representative Autonomous System (AS) level
topologies.
Generating a graph according to the Barabasi-Albert model is a three step process that involves:
1) placing the nodes on the plane (parent class, ASModel handles this entirely)
2) interconnecting the nodes according to their outdegrees.
An Internet topology is represented as graph with the nodes
representing routers (or ASs, depending on the topology level) and
the edges representing links between them.
Places nodes of nodeType (nodes that represent individual ASs in this case) onto the
plane according to the NodePlacement type and add them to our graph g.