Make PRIMO compatible to networkx 2

Created by: jpoeppel

This PR makes some small changes to the use of networkx so that the new version 2 works while still supporting networkx 1.x (only tested the latest 1.11). All tests pass with these changes. I also added a small monkey patch in case 1.x is used since the backward compatible function calls use the list-versions of functions in networkx 1.x while the iterator versions have become standard in 2.0. The monkey patch will override the function names to the iterator versions for 1.x so that we do not have higher memory consumption in this case.

This PR fixes #20 (closed)