this will be used in plane to make AHRS SYS_STATUS unhealthy if a user
tries to fly with EKF enabled without a full 3D accel cal.
Note that it doesn't rely on using AP_Param load() to detect that a
value has been set, as some users are first doing a 3D cal then later
doing a 1D cal. In that case load() was returning true and would give
a false positive
Andrew Tridgell