[GFS] Use Pressure at MSL instead of surface

Pressure at surface level was the wrong value. All barometers and
altimeters use pressure reduced to mean sea level. Reading that value
from GFS instead returns much more reasonable values.

ref T537

src/blackmisc/simulation/fsx/simconnectutilities.cpp

@@ -246,7 +246,7 @@ namespace BlackMisc
                 simconnectMetar += QLatin1String(" Q");
                 // NNNN = altimeter in millibars
                 static const QString arg1s("%1");
-                const auto altimeter = gridPoint.getSurfacePressure().valueInteger(CPressureUnit::mbar());
+                const auto altimeter = gridPoint.getPressureAtMsl().valueInteger(CPressureUnit::mbar());
                 simconnectMetar += arg1s.arg(altimeter, 4, 10, QLatin1Char('0'));
 
                 return simconnectMetar;