ENH: template invariant base classes for {fa,fae,fv,fvs,point}PatchField

- simplifies construction/inheritance

ENH: add {fa,fv}PatchField::zeroGradientType() static

- can be used to avoid literal "zeroGradient" in places

STYLE: adjust naming of pointPatch runtime selection table

- simply use 'patch' as per fa/fv fields

STYLE: add zero-size guard to patch constraintType(const word&)

tutorials/compressible/rhoPorousSimpleFoam/angledDuct/implicit/system/sampling

@@ -41,7 +41,7 @@ cellZoneID
                             fieldName,
                             mesh,
                             dimless,
-                            zeroGradientFvPatchField<scalar>::typeName
+                            fvPatchScalarField::zeroGradientType()
                         )
                     );