Make torus copyable

src/example6.cc

@@ -24,7 +24,7 @@ namespace Curved {
 
 struct Torus
 {
-  double const r1 = 2.0, r2 = 1.0;
+  double r1 = 2.0, r2 = 1.0;
 
   auto operator() (FieldVector<double,2> const& u) const
   {
@@ -71,4 +71,4 @@ int main(int argc, char** argv)
   // 4. Write grid to vtu file
   SubsamplingVTKWriter writer{grid2.leafGridView(), refinementIntervals(3)};
   writer.write("torus2");
-}
\ No newline at end of file
+}