[bugfix] Move to_false_type to namespace Dune::Std::

We decided to have all stl like stuff in Dune::Std:: and not
in Std:: Besides our decision with the previous version tupleutility.hh
did not compile if a header refering to Dune::Std:: was included.

dune/common/std/type_traits.hh

@@ -3,6 +3,9 @@
 
 #include <type_traits>
 
+namespace Dune
+{
+
 namespace Std
 {
 
@@ -73,4 +76,6 @@ namespace Std
 
 } // namespace Std
 
+} // namespace Dune
+
 #endif // #ifndef DUNE_COMMON_STD_TYPE_TRAITS_HH