Read the K-points from file.

def yaml_read_kpoints(filename, doc_tag="!Kpoints"):
    """Read the K-points from file."""
    with YamlTokenizer(filename) as r:
        doc = r.next_doc_with_tag(doc_tag)
        d = yaml.safe_load(doc.text_notag)

        return np.array(d["reduced_coordinates_of_qpoints"])