controls buoancy offset

[MrMondrian]

This is an idea we've been talking about since mid fall, but never got around to doing it mostly because it was suggested by a new member who was committed to doing it then quit before he implemented it.

The z pid is particularly hard to tune because of the buoyant force. If the I term is high then the auv will descend at a good speed but the i term will over wind/up down when away from the setpoint. if too low the auv takes too long to descend. in the worst case, the auv will be unable to stabilize at the setpoint (the happened in our last run at comp and is why dougie didn't go through the gate). adding an additional constant buoyant offset force in superimposer (probably only activate when a goal is active) simplify the z pid to be essentially the same as x and y. tuning would be easy and x y z pids could be reduced from 9 gains to 3