Fix issue related to list conversion

[sameeravithana]

Error: File "../graphite/preprocessing.py", line 45, in pick_edges u, v = G_edges[i] File "../lib/python/site-packages/networkx/classes/reportviews.py", line 929, in __getitem__ u, v = e TypeError: 'int' object is not iterable Fix by converting to lists.

[sameeravithana]

This fix is required for the examples to run with networkx 2.1.