Transition

Added forward mode and 'care free' mode. Also made it possible to not use the reference model in the control, as it is not working out for me. I find it to be very sensitive and not robust if you really want to make the feedforward work. 

I changed the way the current yaw is determined to avoid switching beyond 90 degrees, as this would be unwanted when transitioning (and for quads as well). The other option I considered was changing the BODY_TO_IMU rotation, but I found this to be less clear, for instance when reviewing the logs.

Any comments are welcome!