hRef: Define height in opposite direction to gravity

The reference height is now defined in the direction of -g, whether as
previously it was defined in the direction cmptMag(g). This change makes
the behaviour consistent when the case is transformed. For a "typical"
case with g along one of the negative axes, this should make no
difference. None of the tutorials are affected.

Resolves bug report https://bugs.openfoam.org/view.php?id=2980

applications/solvers/heatTransfer/chtMultiRegionFoam/fluid/createFluidFields.H

@@ -121,13 +121,7 @@ forAll(fluidRegions, i)
         )
     );
 
-    dimensionedScalar ghRef
-    (
-        mag(gFluid[i].value()) > small
-      ? gFluid[i]
-          & (cmptMag(gFluid[i].value())/mag(gFluid[i].value()))*hRefFluid[i]
-      : dimensionedScalar("ghRef", gFluid[i].dimensions()*dimLength, 0)
-    );
+    dimensionedScalar ghRef(- mag(gFluid[i])*hRefFluid[i]);
 
     Info<< "    Adding to ghFluid\n" << endl;
     ghFluid.set