Add Graph method for betweenness centrality

[sbaldu]

I think it would be useful to add methods in the Graph class for calculating the betweenness centrality of a node and maybe also other centrality measures (eigenvalue centrality, closeness centrality, ecc.).
These measures are very useful when studying a network, and this methods are provided by similar libraries (like NetworkX in Python), so I think that it would be a useful addition.