The Characteristic Path Length of a graph is the median of the means of the shortest path lengths
connecting each vertex to all other vertices.   Typical AS level topologies obtained from NLANR exhibit
CPL values between 3.6 - 3.8.

This metric along with the Clustering Coefficient metric measure the small-world properties of a
graph.  Intuitively, a small-world graph is a graph which has a larger clustering coefficient than a
random graph with the same path length.

Contributors:   D.J. Watts, S.H. Stogatz
Reference:      [WS98] Collective Dynamics of "small-world" networks, in Nature 393.
Implemented by: BRITE Development Team
