Multi-rotor simulator.

The library provides a pty emulating the tk3-paparazzi communication protocol
and implements all the tk3-paparazzi features, so that the hardware can be
fully emulated.

The rotor dynamics are simulated using a realistic motor model and a simple
thrust/drag propeller model. The same hardware velocity controller as
implemented in the tk3-mikrokopter software controls the propellers
velocities. Then, a simple free-flying 6D rigid body dynamics simulation can
also be used, although it is not meant as a generic physics engine (e.g. no
collision, no atmosphere, ...).
