docs: 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