def start_trajectory(self, trajectory_id, time_scale=1.0, relative=False,
                         reversed=False, group_mask=ALL_GROUPS):
        """
        starts executing a specified trajectory

        :param trajectory_id: id of the trajectory (previously defined by
               define_trajectory)
        :param time_scale: time factor; 1.0 = original speed;
                                        >1.0: slower;
                                        <1.0: faster
        :param relative: set to True, if trajectory should be shifted to
               current setpoint
        :param reversed: set to True, if trajectory should be executed in
               reverse
        :param group_mask: mask for which CFs this should apply to
        :return:
        """
        self._send_packet(struct.pack('<BBBBBf',
                                      self.COMMAND_START_TRAJECTORY,
                                      group_mask,
                                      relative,
                                      reversed,
                                      trajectory_id,
                                      time_scale))
starts executing a specified trajectory

        :param trajectory_id: id of the trajectory (previously defined by
               define_trajectory)
        :param time_scale: time factor; 1.0 = original speed;
                                        >1.0: slower;
                                        <1.0: faster
        :param relative: set to True, if trajectory should be shifted to
               current setpoint
        :param reversed: set to True, if trajectory should be executed in
               reverse
        :param group_mask: mask for which CFs this should apply to
        :return: