[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/plugins/simulator/fscommon/fsuipcimpl.cpp

@@ -290,7 +290,7 @@ namespace BlackSimPlugin
             NewPress press;
             press.Drift = 0;
             // Pressure is measured in: 16 x mb
-            press.Pressure = static_cast<ushort>(gridPoint.getSurfacePressure().value(CPressureUnit::mbar()) * 16);
+            press.Pressure = static_cast<ushort>(gridPoint.getPressureAtMsl().value(CPressureUnit::mbar()) * 16);
             nw.Press = press;
 
             // writing will take place in