AP_NavEKF : Change mag fusion to use corrected compass values

This enables compass units to be switched in-flight
AP_NavEKF : stop velocity change when flow measurements drop out
AP_NavEKF : Use GPS velocity if PX4Flow sensor fails
AP_NAvEKF : Not For Master

Custom scale factor for testing PX4Flow sensor
AP_NavEKF : Don't estimate focal length scale factor without reliable velocity