More similiarity with the nonlinearity interface

dune/tectonic/globalnonlinearity.hh

@@ -18,6 +18,7 @@ class GlobalNonlinearity {
   using SingletonVectorType = BlockVector<FieldVector<double, 1>>;
 
 public:
+  using IndexSet = Dune::MatrixIndexSet;
   using MatrixType = MatrixTypeTEMPLATE;
   using VectorType = VectorTypeTEMPLATE;
   using LocalMatrixType = typename MatrixType::block_type;