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&)

src/OpenFOAM/fields/pointPatchFields/basic/zeroGradient/zeroGradientPointPatchField.C

@@ -27,6 +27,15 @@ License
 
 #include "zeroGradientPointPatchField.H"
 
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+template<class Type>
+const Foam::word& Foam::pointPatchField<Type>::zeroGradientType()
+{
+    return Foam::zeroGradientPointPatchField<Type>::typeName;
+}
+
+
 // * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
 
 template<class Type>