EKF: Add support for use of GPS heading data.

Heading data is assumed to be from a  dual antenna array at a specified yaw angle offset in body frame, but with the heading data already corrected for antenna offset. The offset is required to apply the correct compensation for combined rotations and to determine when the yaw observation has become badly conditioned.

EKF/estimator_interface.h

@@ -470,6 +470,7 @@ protected:
 	struct map_projection_reference_s _pos_ref {};   // Contains WGS-84 position latitude and longitude (radians) of the EKF origin
 	struct map_projection_reference_s _gps_pos_prev {};   // Contains WGS-84 position latitude and longitude (radians) of the previous GPS message
 	float _gps_alt_prev{0.0f};	// height from the previous GPS message (m)
+	float _gps_yaw_offset{0.0f};	// Yaw offset angle for dual GPS antennas used for yaw estimation (radians).
 
 	// innovation consistency check monitoring ratios
 	float _yaw_test_ratio{0.0f};          // yaw innovation consistency check ratio