using iGraph for python

[michaelkkim]

Look at it Tuesday with Iain

Notes:
-G.add_edge() and G.add_edges() add the edges by the indices of the list of vertices, NOT by the vertex names --really dumb
-I tried replicating this: https://gist.github.com/rbnvrw/c2424fe3ff812da892a0
although this led to the correct number of edges, 3 million more nodes/cases were added somehow
-check if iGraph for python contains algorithms to do centrality measures