[localevaluation.elliptic] fix for single tensor case

dune/gdt/localevaluation/elliptic.hh

@@ -100,7 +100,7 @@ private:
   struct Helper<false, true, anything>
   {
     typedef Stuff::Functions::Constant<E, D, d, D, 1, 1> FactorType;
-    typedef DiffusionTensorImp TensorType;
+    typedef DiffusionFactorImp TensorType;
   };
 
   static const bool single_factor_given = DiffusionFactorImp::dimRange == 1