<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def extract_master_tarball(cls):
 """Unpack tarball of tarballs """

        if not os.path.exists(cls.master_tarball_file):
            cls.download_grids()

        with tarfile.open(os.path.join(ISOCHRONES, cls.master_tarball_file)) as tar:
            logging.info('Extracting {}...'.format(cls.master_tarball_file))
            tar.extractall(ISOCHRONES)