ENH: extrapolateInternal() method for patch fields

- provides a more succinct way of writing

      {fa,fv}PatchField<Type>::patchInternalField(*this)

  as well as a consistent naming that can be used for patches derived
  from valuePointPatchField

ENH: readGradientEntry helper method for fixedGradient conditions

- simplifies coding and logic.
- support different read construct modes for fixedGradient

etc/codeTemplates/dynamicCode/fixedValuePointPatchFieldTemplate.H

@@ -89,8 +89,8 @@ public:
         (
             const pointPatch&,
             const DimensionedField<${TemplateType}, pointMesh>&,
-            const dictionary&,
-            const bool valueRequired=true
+            const dictionary& dict,
+            IOobjectOption::readOption requireValue = IOobjectOption::MUST_READ
         );
 
         //- Construct by mapping a copy onto a new patch