compactCycle
compacts all the nodes in the cycle to a single node called
supernode. The function uses the first and the last node
of the cycle as a fusion point and obtains the costs of the
incoming and outgoing arcs of the new node.
compactCycle(nodes, arcs, cycle)
compactCycle
returns the nodes and the list of arcs
forming a new graph with the compressed cycle within a
supernode. Also returns a list of the correspondences
between the nodes of the new graph and the nodes of the
previous graph.