docs: def _check_data_port_id(self, data_port):
          """Checks the validity of a data port id
  
          Checks whether the id of the given data port is already used by anther data port (input, output) within the
          state.
  
          :param rafcon.core.data_port.DataPort data_port: The data port to be checked
          :return bool validity, str message: validity is True, when the data port is valid, False else. message gives
              more information especially if the data port is not valid
          """
          for input_data_port_id, input_data_port in self.input_data_ports.items():
              if data_port.data_port_id == input_data_port_id and data_port is not input_data_port:
                  return False, "data port id already existing in state"
          for output_data_port_id, output_data_port in self.output_data_ports.items():
              if data_port.data_port_id == output_data_port_id and data_port is not output_data_port:
                  return False, "data port id already existing in state"
          return True, "valid"