FIXED 2 BUGS. NEW FEATURE reading graph from db given graph_id
Bugs:
- nodes TreeMap in read_graph_from_file contained one letter res codes, now three letter codes
- in getCM was passing residues as a TreeMap of nums to three letter residue types, now passing one letter residue types
New constructor for getting graph from db given graph_id, new method to go with that get_db_graph_info