Additional interactome pre-processing functions

[agitter]

#120 adds a new interactome module for edge preprocessing, mainly managing edge directions. We could expand this to include additional operations like removing self edges or converting pairs of directed edges (u,v) (v,u) to undirected edges. Then a user could request those transformations in the dataset portion of the config file. That would help run MEO, which is picky about the types of edges in the interactome (#47).