GPS Yaw: fix heading initialisation and add unit tests

When the antennas are not parallel to the x body axis, the GPS message
contains the angular offset but the data is already corrected in the
driver. EKF2 should then not add this offset during the initialisation.

test/sensor_simulator/gps.cpp

@@ -58,6 +58,10 @@ void Gps::setYaw(float yaw) {
 	_gps_data.yaw = yaw;
 }
 
+void Gps::setYawOffset(float yaw_offset) {
+	_gps_data.yaw_offset = yaw_offset;
+}
+
 void Gps::setFixType(int n)
 {
 	_gps_data.fix_type = n;