Read the list of irreducible perturbations from file.

def yaml_read_irred_perts(filename, doc_tag="!IrredPerts"):
    """Read the list of irreducible perturbations from file."""
    with YamlTokenizer(filename) as r:
        doc = r.next_doc_with_tag(doc_tag)
        d = yaml.safe_load(doc.text_notag)

        return [AttrDict(**pert) for pert in d["irred_perts"]]