Motor model and MAVLink interface improvements

[junwoo091400]

Work in progress!

Improvements

1. Previously the PID based force was always being applied to velocity controlled joints (basically all rotors / motors), but this shouldn't be the case as the motor_model can correctly set the thrust force on it's own

Comment

I was trying out the boat model and was playing around with the left/right thrusters, and while trying to convert them into the ignition hydrodynamics model, I realized that there are some improvements to be made in the models in the repo.