Immunogenicity ligandome

Instead of trying to model the space of immunogenic epitope strings directly, instead let's model immunogenicity by only keeping track of which patterns *can't* be recognized by T-cells. These are epitopes which overlap with the set of "self" peptides used for negative selection in the thymus. 
For my own educationing: any particular reason you chose to say "data_path = None" vs. "data_path = DEFAULT_PEPTIDE_DIR" here?