Add scaling for custom MultiTypeBlockMatrix type.

dune/solvers/test/energynormtest.cc

@@ -143,6 +143,8 @@ struct NestableMultiTypeBlockVector : public Dune::MultiTypeBlockVector<Args...>
 template<class... Args>
 struct NestableMultiTypeBlockMatrix : public Dune::MultiTypeBlockMatrix<Args...> {
   constexpr static size_t blocklevel = 1; // fake value needed for BCRSMatrix nesting
+  template<class K, typename = std::enable_if_t<Dune::IsNumber<K>::value>>
+  void operator*=(K v) { Dune::Hybrid::forEach(*this, [v](auto& b) { b *= v; }); } // allow multiplication by scalar
 };
 
 // inject field traits for custom multi type block vector to be used by the norms