def _read_configuration(config_filename):
        """
        Checks the supplement file.

        :param str config_filename: The name of the configuration file.

        :rtype: (configparser.ConfigParser,configparser.ConfigParser)
        """
        config = ConfigParser()
        config.read(config_filename)

        if 'supplement' in config['database']:
            path = os.path.dirname(config_filename) + '/' + config.get('database', 'supplement')
            config_supplement = ConfigParser()
            config_supplement.read(path)
        else:
            config_supplement = None

        return config, config_supplement
Checks the supplement file.

        :param str config_filename: The name of the configuration file.

        :rtype: (configparser.ConfigParser,configparser.ConfigParser)