msg: Add heading data to GPS blending output log data

msg/ekf_gps_position.msg

@@ -14,4 +14,6 @@ float32 vel_e_m_s		# GPS East velocity, (metres/sec)
 float32 vel_d_m_s		# GPS Down velocity, (metres/sec)
 bool vel_ned_valid		# True if NED velocity is valid
 uint8 satellites_used		# Number of satellites used
+float32 heading			# heading angle of XYZ body frame rel to NED. Set to NaN if not available and updated (used for dual antenna GPS), (rad, [-PI, PI])
+float32 heading_offset		# heading offset of dual antenna array in body frame. Set to NaN if not applicable. (rad, [-PI, PI])
 uint8 selected			# GPS selection: 0: GPS1, 1: GPS2. 2: GPS1+GPS2 blend