AP_NavEKF: Prevent touchdown baro errors tripping height innovation check

Ground effect baro errors can cause a spike in height innovation on disarming if ground effect compensation was used during the landing. This causes a transient AHRS fault message if this innovation is outside the pre-arm check limits.
Resetting the vertical position state to the measured height after disarming prevents this.

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -4809,6 +4809,8 @@ void NavEKF::performArmingChecks()
             // initialise filtered altitude used to provide a takeoff reference to current baro on disarm
             // this reduces the time required for the filter to settle before the estimate can be used
             meaHgtAtTakeOff = hgtMea;
+            // reset the vertical position state to faster recover from baro errors experienced during touchdown
+            state.position.z = -hgtMea;
         } else if (_fusionModeGPS == 3) { // arming when GPS useage has been prohibited
             if (optFlowDataPresent()) {
                 PV_AidingMode = AID_RELATIVE; // we have optical flow data and can estimate all vehicle states