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. 
It's done by the same script that removes duplicate peptide sets: https://github.com/iskandr/mhc_ligandome/blob/master/reduce_peptide_sets.py