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