[bugfix] Add missing specialization FieldTraits<MultiTypeBlockMatrix>

Notice that this also fixes an incorrect `MultiTypeBlockMatrix::field_type`
in a special setting: If you have a nested matrix
`MultiTypeBlockMatrix< MultiTypeBlockVector<MultiTypeBlockMatrix<...>>>`,
the outer `MultiTypeBlockMatrix::field_type` forwards to the `MultiTypeBlockVector::field_type`
which forwards to the inner `FieldTraits<MultiTypeBlockMatrix>::field_type`.

dune/istl/multitypeblockmatrix.hh

@@ -404,6 +404,18 @@ namespace Dune {
 
   };
 
+
+  /** @addtogroup DenseMatVec
+      @{
+   */
+  template <typename... Rows>
+  struct FieldTraits< MultiTypeBlockMatrix<Rows...> >
+  {
+    using field_type = typename MultiTypeBlockMatrix<Rows...>::field_type;
+    using real_type  = typename FieldTraits<field_type>::real_type;
+  };
+
+
   /**
      @brief << operator for a MultiTypeBlockMatrix