[bugfix]Implement tuple protocol for custom type in resizetest.cc

This fixes some fallout of the removal of `MultiTypeBlockMatrix::size()`:
By not providing a generic container interface you can no longer use generic
algorithms like indexed for-loops for matrices. One way to work around
this is to implement the `tuple` protocol to make `Hybrid::size()` work again.

For an alternative solution cf. core/dune-common!1209

dune/matrix-vector/test/resizetest.cc

@@ -41,6 +41,26 @@ namespace Dune { namespace MatrixVector { namespace Traits {
   };
 }}}
 
+// Implement tuple protocol for custom matrix
+namespace std
+{
+  template <size_t i, typename... Args>
+  struct tuple_element<i,CustomMultiTypeBlockMatrix<Args...> >
+  {
+    using type = typename std::tuple_element<i, std::tuple<Args...> >::type;
+  };
+
+  /** \brief Make std::tuple_size work for MultiTypeBlockMatrix
+   *
+   * It derives from std::tuple after all.
+   */
+  template <typename... Args>
+  struct tuple_size<CustomMultiTypeBlockMatrix<Args...> >
+    : std::integral_constant<std::size_t, sizeof...(Args)>
+  {};
+}
+
+
 
 class ResizeTestSuite {
   /// typedefs