Surface Tracking

Clyde is the one working on it.

"Surftrak: I added a commit to use the barometer delta to adjust the rangefinder target. Then I also added a commit to expose the rangefinder controls in Lua, so I can build a Lua script that allows you to set / increment / decrement the rf target using the joystick. I have done zero testing in the water. Testing in SITL looks OK. So pretty bleeding-edge...."