Provide FieldTraits that are missing in the core modules.

dune/matrix-vector/traits/CMakeLists.txt

 #install headers
 install(FILES
-  scalartraits.hh
+  fieldtraits.hh
   matrixtraits.hh
-  vectortraits.hh
+  scalartraits.hh
   utilities.hh
+  vectortraits.hh
   DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/matrix-vector/traits)

dune/matrix-vector/traits/fieldtraits.hh

+#ifndef DUNE_MATRIX_VECTOR_TRAITS_FIELDTRAITS_HH
+#define DUNE_MATRIX_VECTOR_TRAITS_FIELDTRAITS_HH
+
+#include <dune/common/typetraits.hh>
+#include <dune/istl/scaledidmatrix.hh>
+
+/**
+ * \file
+ * Add FieldTraits that are not in the core modules.
+ */
+
+namespace Dune {
+
+template <class K, int n>
+struct FieldTraits<ScaledIdentityMatrix<K, n>> {
+  using field_type = field_t<K>;
+  using real_type = real_t<K>;
+};
+
+} // end namespace Dune
+
+#endif // DUNE_MATRIX_VECTOR_TRAITS_FIELDTRAITS_HH