Barometer and Sonar SITL sensor enhancement

There has been a SIM_BARO_RND parameter for a while that's never been wired up.  The first commit wires this up and adds a drift rate parameter, between which you can simulate pretty accurately how baros behave

Secondly add support for simulating a downward-looking sonar sensor, including noise and probabilistic glitching to max distance.  At the moment only supports simulating the Maxbotics XL.