With this commit we fix a few occurrences of loop variables being
accessed in closures.
While we touch the code, we also migrate it to using the require
library.
This commit adds betweenness centrality to the available node metrics.
Betweenness centrality is a per node centrality measure which for an
arbitrary node v equals to the sum of shortest paths going trough v
divided by the number of all shortest paths for for each vertex pair
k, s where k != s != v.
