VTK output modules: make the processElement() method a dummy if the VTK output is disabled

an interesting side effect is that the Newton convergence output is
only defined if the VTK output is enabled.

Also, this patch could be implemented more efficiently by retrieving
the EnableVtkOutput only once per output module (instead of once per
element).

applications/ebos/ecloutputblackoilmodule.hh

@@ -158,6 +158,9 @@ public:
      */
     void processElement(const ElementContext &elemCtx)
     {
+        if (!EWOMS_GET_PARAM(TypeTag, bool, EnableEclOutput))
+            return;
+
         typedef Opm::MathToolbox<Evaluation> Toolbox;
 
         if (!std::is_same<Discretization, Ewoms::EcfvDiscretization<TypeTag> >::value)