Port filter from python to cpp

[oyeb]

Constraints

• struct quad_state is maintained by saber.cpp.
• saber.cpp includes this module and calls filter(&quad_state, mpu_GetReadings()).
• attitude.cpp must maintain it's locals just as attitude.py does.
• In the end, this module should provide
  • filtered rotational velocities, linear accelerations.
  • estimated roll, pitch and yaw angles. We don't need roll, pitch, yaw axis vectors, that's only needed on the ground_station for visualisation.

Start by just dumping everything into quad_state without filtering. Then add filter().