make data-members of continuous datacollector protected

dune/vtk/datacollectors/continuousdatacollector.hh

@@ -136,7 +136,7 @@ public:
     return data;
   }
 
-public: // protected:
+protected:
   using Super::gridView_;
   std::uint64_t numPoints_ = 0;
   std::uint64_t numCells_ = 0;