GPS: Add height above the ellipsoid info

Modification of the GPS message, add height above the ellipsoid info.

msg/vehicle_gps_position.msg

@@ -3,6 +3,7 @@ uint64 timestamp_position	# Timestamp for position information
 int32 lat			# Latitude in 1E-7 degrees 
 int32 lon			# Longitude in 1E-7 degrees 
 int32 alt			# Altitude in 1E-3 meters (millimeters) above MSL  
+int32 alt_ellipsoid # Altitude in 1E-3 meters (millimeters) above Ellipsoid
 
 uint64 timestamp_variance
 float32 s_variance_m_s		# speed accuracy estimate m/s 

src/drivers/gps/ubx.cpp

@@ -874,6 +874,7 @@ UBX::payload_rx_done(void)
 		_gps_position->alt	= _buf.payload_rx_nav_posllh.hMSL;
 		_gps_position->eph	= (float)_buf.payload_rx_nav_posllh.hAcc * 1e-3f; // from mm to m
 		_gps_position->epv	= (float)_buf.payload_rx_nav_posllh.vAcc * 1e-3f; // from mm to m
+		_gps_position->alt_ellipsoid = _buf.payload_rx_nav_posllh.height;
 
 		_gps_position->timestamp_position = hrt_absolute_time();