docs: def set_features(self, filter_type):
          """Calls splitter to split percolator output into target/decoy
          elements.
          Writes two new xml files with features. Currently only psms and
          peptides. Proteins not here, since one cannot do protein inference
          before having merged and remapped multifraction data anyway.
          """
          elements_to_split = {'psm': self.allpsms, 'peptide': self.allpeps}
          self.features = self.splitfunc(elements_to_split, self.ns, filter_type)