Blimp: Add motor-based blimp

[MichelleRos]

This adds a new Blimp frame that uses four bidirectional motors on the gondola to get 4DOF movement (X,Y,Z linear movement plus Yaw).

This has been tested in the SITL frame added in this PR, as well as on a real blimp I've built (but that was before the parameter move...). Will test at least in SITL again before marking it as ready.