Speed up creation of random graphs

[Islast]

To calculate global measures, we generate a cohort of random graphs via edge-swapping. This is very slow ~ 2 minutes for 100 random graphs, where we need about 10,000. For situations where we have to create a cohort of random graphs each for a group of graphs, this is too slow.

[Islast]

I'm going to try using graph_tool's edge swapping function graph_tool.generation.random_rewire

[Islast]

The snag with graph-tool is that it's a pain to install