Add a nonlinear trajectory tracking controller.

[whoenig]

To test, follow these steps:

1. compile the firmware with ESTIMATOR set to "kalman" and CONTROLLER set to "mellinger".
2. roslaunch crazyflie_demo external_position_vicon.launch uri:=radio://0/100/2M/E7E7E7E728
3. In "crazyflie_ros/crazyflie_demo/scripts": python execute_trajectory.py figure8withTakeoffAndLanding.csv

The controller can also be used with manual flight

[ataffanel]

Thanks a lot for the PR! We plan to look at it on Friday (the flight lab is a bit busy right now ... :).

[ataffanel]

Thanks, it works fine with a Qualisys system too.
One question: at the end of the trajectory, the CF is held running at (0,0,0) and if I stop the execute_trajectory script with ctrl-C the CF will jump away. Is that something you have observed?

[whoenig]

Thanks! I've seen the issue and I am working on a fix for that. I already found the root cause: The controller doesn't handle the "timeout" setpoint sent here well. I hope that I can address it in the controller itself without changing the setpoint logic, but I am not sure yet.

[whoenig]

Update: I was able to fix the CTRL+C problem in controller_mellinger alone. I put a proposed fix for the issue in another PR: #288. I'll fix the other issue (CF not landing without CTRL+C) in crazyflie_ros.
What changes did you have to do to support Qualisys?