Returns the name of an intermediate file for storing features.

def _feature_file(self, parallel = None, index = None):
    """Returns the name of an intermediate file for storing features."""
    if index is None:
      index = 0 if parallel is None or "SGE_TASK_ID" not in os.environ else int(os.environ["SGE_TASK_ID"])
    return os.path.join(self.feature_directory, "Features_%02d.hdf5" % index)