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.