Compute betweenness centrality for a subgraph.
Enchanced version of the method in centrality module that allows specifying a list of sources (subgraph).
weighted_edges:: consider edge weights by running Dijkstra’s algorithm (no effect on unweighted graphs).
sources:: list of nodes to consider as subgraph
See Sec. 4 in 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
This algorithm does not count the endpoints, i.e. a path from s to t does not contribute to the betweenness of s and t.