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.H

@@ -34,8 +34,8 @@ SourceFiles
 
 \*---------------------------------------------------------------------------*/
 
-#ifndef zeroGradientPointPatchField_H
-#define zeroGradientPointPatchField_H
+#ifndef Foam_zeroGradientPointPatchField_H
+#define Foam_zeroGradientPointPatchField_H
 
 #include "pointPatchField.H"
 
@@ -53,7 +53,6 @@ class zeroGradientPointPatchField
 :
     public pointPatchField<Type>
 {
-
 public:
 
     //- Runtime type information