[type_traits] add printing of StencilType::automatic

dune/gdt/type_traits.hh

@@ -69,6 +69,8 @@ inline std::ostream& operator<<(std::ostream& out, const Stencil& stencil)
     out << "intersection";
   else if (stencil == Stencil::element_and_intersection)
     out << "element_and_intersection";
+  else if (stencil == Stencil::automatic)
+    out << "automatic";
   else
     out << "Stencil(" << int(stencil) << ")";
   return out;