HoverThrustEstimator: add a new single state estimator

with measurement noise auto-tuning

The purpose of this estimator is to improve land detection and vertical
velocity feedforward

Recovery strategy:
This is required when the setpoint suddenly changes in air or that the
EKF is diverging. A lowpassed test ratio is used as a trigger for the recovery logic
Also, a lowpassed residual is used to estimate the steady-state value
and remove it when estimating the accel noise to avoid increasing the
accel noise when the redisual is caused by an offset.

msg/CMakeLists.txt

@@ -68,6 +68,7 @@ set(msg_files
 	gps_dump.msg
 	gps_inject_data.msg
 	home_position.msg
+	hover_thrust_estimate.msg
 	input_rc.msg
 	iridiumsbd_status.msg
 	irlock_report.msg