Document how to fix random seed

[jboynyc]

That's important for reproducibility.

[jboynyc]

igraph just uses Python's rng, so import random; random.seed(1234) ought to be enough.

But leidenalg requires the seed to be passed as an argument to find_partition. Should a fixed seed be hard-coded?

[jboynyc]

Something like the rcParams system in matplotlib might be the best way to set this and other global parameters.