Nodes and edges takes a lot of time

[edward-mich]

Small performance issue I think — when I have multiple lists (say over 8-10), the get_nodes_and_edges and get_nonzero_nodes_and_eges takes a lot of time to run, probably because it takes all possible intersections (2**n_sets), ones which are not even present in the dataset. Any workarounds?

[hshhrr]

Will look into it. Thanks for pointing it out.