docs: def _read_prm_file(prm_filename):
      """read the prm file"""
      logger.debug("Reading config-file: %s" % prm_filename)
      try:
          with open(prm_filename, "r") as config_file:
              prm_dict = yaml.load(config_file)
      except yaml.YAMLError:
          raise ConfigFileNotRead
      else:
          _update_prms(prm_dict)