ENH: DimensionedField, GeometricField enhancements and style changes

- constCast() convenience methods.
  This simplies coding in places, especially in solvers

- inline some methods within headers, for better overview/maintenance

- only set DimensionedField default value on construct if not read

- storeOldTimes() does not require const_cast

applications/solvers/multiphase/compressibleInterFoam/twoPhaseMixtureThermo/twoPhaseMixtureThermo.H

@@ -135,7 +135,7 @@ public:
             virtual volScalarField& he()
             {
                 NotImplemented;
-                return const_cast<volScalarField&>(volScalarField::null());
+                return volScalarField::null().constCast();
             }
 
             //- Enthalpy/Internal energy [J/kg]

applications/solvers/multiphase/compressibleMultiphaseInterFoam/multiphaseMixtureThermo/multiphaseMixtureThermo.H

@@ -243,7 +243,7 @@ public:
             virtual volScalarField& he()
             {
                 NotImplemented;
-                return const_cast<volScalarField&>(volScalarField::null());
+                return volScalarField::null().constCast();
             }
 
             //- Enthalpy/Internal energy [J/kg]

applications/solvers/multiphase/interCondensatingEvaporatingFoam/temperaturePhaseChangeTwoPhaseMixtures/twoPhaseMixtureEThermo/twoPhaseMixtureEThermo.H

@@ -86,7 +86,7 @@ public:
         virtual volScalarField& he()
         {
             NotImplemented;
-            return const_cast<volScalarField&>(volScalarField::null());
+            return volScalarField::null().constCast();
         }
 
         //- Return access to the internal energy field [J/Kg]