ekf2: add unaided_yaw for more resilient yaw control

This estimate doesn't converge to the true yaw but can be used as a
more consistent but drifting heading source.
It can be used by a setpoint generator to adjust its heading setpoint
while the true yaw estimate is converging in order to keep a constant
course over ground.

src/modules/ekf2/EKF/output_predictor.h

@@ -94,6 +94,9 @@ public:
 
 	const matrix::Quatf &getQuaternion() const { return _output_new.quat_nominal; }
 
+	// get a yaw value solely based on bias-removed gyro integration
+	float getUnaidedYaw() const { return _unaided_yaw; }
+
 	// get the velocity of the body frame origin in local NED earth frame
 	matrix::Vector3f getVelocity() const { return _output_new.vel - _vel_imu_rel_body_ned; }
 
@@ -184,6 +187,8 @@ private:
 
 	matrix::Vector3f _imu_pos_body{};                ///< xyz position of IMU in body frame (m)
 
+	float _unaided_yaw{};
+
 	// output complementary filter tuning
 	float _vel_tau{0.25f};                   ///< velocity state correction time constant (1/sec)
 	float _pos_tau{0.25f};                   ///< position state correction time constant (1/sec)