RobotBackend should be cleaned up

[wumanu]

the loop of the RobotBackend could potentially be simplified a bit, experience shows that the logic now is a little error prone.

for instance, i think there is a slight problem in this part:

            if (std::isfinite(
                    robot_driver_.get_max_inter_action_duration_s()) &&
                robot_data_->desired_action->newest_timeindex() < t)
            {
                uint32_t action_repetitions =
                    robot_data_->status->newest_element().action_repetitions;

                if (action_repetitions < max_action_repetitions_)
                {
                    robot_data_->desired_action->append(
                        robot_data_->desired_action->newest_element());
                    status.action_repetitions = action_repetitions + 1;
                }
            }

There is a (small) chance that we check the if-clause and realize that action t has not been appended yet, so we enter the loop, but before we append the repeated action, the user in fact appends another action.

while this problem is most likely practically irrelevant, it hints at some logical issues. not sure what's the best solution, maybe the timeseries should have a lock function?