Solvers based on p_rgh: Added support for optional hRef

Allows the specification of a reference height, for example the height
of the free-surface in a VoF simulation, which reduces the range of p_rgh.

hRef is a uniformDimensionedScalarField specified via the constant/hRef
file, equivalent to the way in which g is specified, so that it can be
looked-up from the database.  For example see the constant/hRef file in
the DTCHull LTSInterFoam and interDyMFoam cases.

src/finiteVolume/cfdTools/general/include/readhRef.H

@@ -0,0 +1,13 @@
+    Info<< "\nReading hRef" << endl;
+    uniformDimensionedScalarField hRef
+    (
+        IOobject
+        (
+            "hRef",
+            runTime.constant(),
+            mesh,
+            IOobject::READ_IF_PRESENT,
+            IOobject::NO_WRITE
+        ),
+        dimensionedScalar("hRef", dimLength, 0)
+    );