remove unused variable warning

dune/vtk/gridcreators/continuousgridcreator.hh

@@ -47,7 +47,7 @@ namespace Dune
         for (std::size_t i = 0; i < types.size(); ++i) {
           auto type = Vtk::to_geometry(types[i]);
           Vtk::CellType cellType{type};
-          auto refElem = referenceElement<double,Grid::dimension>(type);
+          [[maybe_unused]] auto refElem = referenceElement<double,Grid::dimension>(type);
 
           int nNodes = offsets[i] - (i == 0 ? 0 : offsets[i-1]);
           assert(nNodes == refElem.size(Grid::dimension));