hector_quadrotor_controller: improved automatic landing detection and shutdown on rollovers
hector_quadrotor_gazebo_plugins: publish propulsion and aerodynamic wrench as WrenchStamped

This is primarily for debugging purposes.
The default topic for the propulsion plugin has been changed to propulsion/wrench.
hector_quadrotor_model: disabled quadratic term (CT2s) in propeller speed to performance factor J

Because of this term the model output a non-zero thrust even if the propeller speed was zero.
The quadratic term is due to drag and therefore covered in the aerodynamics model.
hector_quadrotor_model: make models more robust against irregular input values

Especially on collisions with walls and obstacles Gazebo can output very high
velocity values for some reason, which caused the propulsion and aerodynamic
models to output infinite values or NaN as resulting forces and torques, with
the consequence that Gazebo effectively stopped simulation and showed the quadrotor
at the origin of the world frame.

With this patch the simulation is much more stable in case of collisions or
hard landings.