DOC: elaborate the usage of function objects

ENH: update libs of etc/caseDicts/postProcess items
  ENH: ensure destructor=default
  ENH: ensure constness
  ENH: ensure no 'copy construct' and 'no copy assignment' exist
  TUT: add examples of function objects with full set
       of settings into a TUT if unavailable
  TUT: update pisoFoam/RAS/cavity tutorial in terms of usage

src/functionObjects/field/zeroGradient/zeroGradient.C

@@ -39,13 +39,7 @@ namespace Foam
 namespace functionObjects
 {
     defineTypeNameAndDebug(zeroGradient, 0);
-
-    addToRunTimeSelectionTable
-    (
-        functionObject,
-        zeroGradient,
-        dictionary
-    );
+    addToRunTimeSelectionTable(functionObject, zeroGradient, dictionary);
 }
 }