Compute betweenness centrality for nodes.
Betweenness centrality is the fraction of number of shortests paths that pass through each node.
The keyword normalized (default=True) specifies whether the betweenness values are normalized by b=b/(n-1)(n-2) where n is the number of nodes in G.
The keyword weighted_edges (default=False) specifies whether to use edge weights (otherwise weights are all assumed equal).
The algorithm is from Ulrik Brandes, A Faster Algorithm for Betweenness Centrality. Journal of Mathematical Sociology 25(2):163-177, 2001. http://www.inf.uni-konstanz.de/algo/publications/b-fabc-01.pdf