[boost] [Graph] Pull request for multiplicities support in brandes_betweenness_centrality

I've put a pull request for ticket #9935 https://svn.boost.org/trac/boost/ticket/9935 https://github.com/boostorg/graph/pull/6 Explanations included in the ticket and the pull request. I can try to make the implementation for graph_parallel if this get accepted. -- Javier Dehesa