implement explicit prefetching

the main part of this patch is actually an API change of
Ewoms::ThreadedEntityIterator to faciliate the change (since we should
prefetch the data for the element which is to be linarized after the
current one.)  Also, this patch is only an 80% solution: it would be
good if the cached intensive quantities of the model and the stencil
of an element could also be prefetched, but that's currently not
easily possible in a performant way because iterating over the
intersections of an element negates the performance gains obtained by
prefetching.

applications/ebos/eclproblem.hh

@@ -346,6 +346,9 @@ public:
         updatePffDofData_();
     }
 
+    void prefetch(const Element& elem) const
+    { pffDofData_.prefetch(elem); }
+
     /*!
      * \brief This method restores the complete state of the well
      *        from disk.

applications/ebos/eclwellmanager.hh

@@ -407,11 +407,8 @@ public:
 #endif
         {
             ElementContext elemCtx(simulator_);
-            auto elemIt = simulator_.gridManager().gridView().template begin</*codim=*/0>();
-            for (threadedElemIt.beginParallel(elemIt);
-                 !threadedElemIt.isFinished(elemIt);
-                 threadedElemIt.increment(elemIt))
-            {
+            auto elemIt = threadedElemIt.beginParallel();
+            for (; !threadedElemIt.isFinished(elemIt); elemIt = threadedElemIt.increment()) {
                 const Element& elem = *elemIt;
                 if (elem.partitionType() != Dune::InteriorEntity)
                     continue;