Citation trees

The function in citationTrees.py creates a single-level citation network map. Including more levels are very slow: there's a commented chunk of code that should in principle do this, but I kept getting timeout errors. I think this is because composing the graphs becomes slow as the graphs become larger, but I'm not sure about this. I used networkX python module for mapping, and matplotlib for viewing.

Todo: 
1. add labels to the nodes/edges. Right now red color represents groups of citations, and blue represents individual papers. Edges represent links between the groups/papers. 
2. It might be better to use a directed graph instead of an undirected graph.