Returns True if the mapping is injective (1-to-1).

def is_injective(self):
        '''Returns True if the mapping is injective (1-to-1).'''
        codomain_residues = [v.to_pdb_residue_id for k, v in self.mapping.iteritems()]
        return(len(codomain_residues) == len(set(codomain_residues)))