this buffers up _omega_I changes in _omega_I_sum over a period of 10
seconds, applying the slope limit only when _omega_I_sum is
transferred to _omega_I.
The result is a huge improvement in the ability of _omega_I to track
gyro drift over the long term.
when a user first connects with USB, and later switches to the
telemetry port without restarting we were getting zero for error_yaw
in the logs, as AHRS.get_error_yaw() was being called twice.
This ensures we give the last value after the counter is reset
Andrew Tridgell