[bugfix] Fix row type of MultiTypeBlockMatrix

This was using a matrices as rows which should
never have worked. Surprisingly it only failed
with the recent modifications on how `field_type`
is computed in dune-istl.

dune/solvers/test/energynormtest.cc

@@ -234,8 +234,8 @@ int main() try
   /// test with multi blocked vectors
   // types
   using MVector = CustomMultiTypeBlockVector<BVector, BVector>;
-  using MMatrix0 = CustomMultiTypeBlockMatrix<BMatrix, BMatrix>;
-  using MMatrix1 = CustomMultiTypeBlockMatrix<BMatrix, BMatrix>;
+  using MMatrix0 = CustomMultiTypeBlockVector<BMatrix, BMatrix>;
+  using MMatrix1 = CustomMultiTypeBlockVector<BMatrix, BMatrix>;
   using MMatrix = CustomMultiTypeBlockMatrix<MMatrix0, MMatrix1>;
   // instance setup
   using namespace Dune::Hybrid;
@@ -249,8 +249,8 @@ int main() try
   /// test with blocked multitype vectors
   // types
   using NVector = CustomMultiTypeBlockVector<FVector, FVector>;
-  using NMatrix0 = CustomMultiTypeBlockMatrix<FMatrix, FMatrix>;
-  using NMatrix1 = CustomMultiTypeBlockMatrix<FMatrix, FMatrix>;
+  using NMatrix0 = CustomMultiTypeBlockVector<FMatrix, FMatrix>;
+  using NMatrix1 = CustomMultiTypeBlockVector<FMatrix, FMatrix>;
   using NMatrix = CustomMultiTypeBlockMatrix<NMatrix0, NMatrix1>;
   constexpr size_t bnSize = 4;
   using BNVector = BlockVector<NVector>;