code: def prefixes_to_fns(self, prefixes: List[str]) -> Tuple[List[str], List[str]]:
          
          # TODO Return pathlib.Paths
          feat_fns = [str(self.feat_dir / ("%s.%s.npy" % (prefix, self.feat_type)))
                      for prefix in prefixes]
          label_fns = [str(self.label_dir / ("%s.%s" % (prefix, self.label_type)))
                        for prefix in prefixes]
          return feat_fns, label_fns
docs: Fetches the file paths to the features files and labels files
          corresponding to the provided list of features