EKf3: control scaling while using Optical Flow leads to drift in high winds

[rmackay9]

When the EKF uses optical flow it provides a control scaler which is used by the position controller to limit the velocity feed-forward and acceleration. This works OK in calm conditions but if the wind is strong enough the control outputs are not enough to allow the vehicle to control its position and it drifts away

This can be reproduced in SITL by doing the following:

• use sim_vehicle.py to start a Copter
• load the copter-optflow.parm file and restart SITL (some instructions on the wiki here)
• param set RNGFND1_MAX_CM 7000
• param set SIM_WIND_SPD 10
• arm and takeoff to 50m and note that the vehicle drifts
• map set showsimpos 1 (<-- to prove that the EKF is estimating the position correctly, it's the controller's that can't keep up)

AC_PositionControl consumes the EKF3 control limits here

This issue is based on this discussion

To solve this issue we have at least three choices:

• add a bit to an option parameter to allow disabling the control limiting. PSC_OPTIONS (which doesn't exist yet) would be the most obvious place but other alterantives include FLOW_OPTIONS, EK3_SRC_OPTIONS or EK3_OPTIONS.
• completely remove the EKF3 feature to impact the position controller scaling. This is very old code and it's unusual for the control and estimation systems to tell each other what to do
• find a better method for the position controller to limit its response so that the flow sensor is not overwhelmed. this could include moving some code out of the EKF