[elliptic-sipdg-discretization] readd current_grid_width()

- without it, compiler complains about pure virtual function

test/elliptic-sipdg-discretization.hh

@@ -12,6 +12,8 @@
 
 #include <dune/common/timer.hh>
 
+#include <dune/fem/misc/gridwidth.hh>
+
 #include <dune/stuff/common/memory.hh>
 #include <dune/stuff/grid/boundaryinfo.hh>
 #include <dune/stuff/la/container.hh>
@@ -291,6 +293,12 @@ public:
     return test_.level_grid_part(current_level_).indexSet().size(0);
   }
 
+  virtual double current_grid_width() const override
+  {
+    assert(current_level_ < test_.num_levels());
+    return Dune::Fem::GridWidth::calcGridWidth(test_.level_grid_part(current_level_));
+  }
+
   virtual double compute_on_current_refinement() override final
   {
     using namespace Dune;