added nullopt, in_place and bad_optional_access from std namespace to Dune::Std namespace

Summary

Import types std::nullopt_t, std::in_place_t, and std::bad_optional_access and instances nullopt and in_place of these types into namespace Dune::Std in the file <dune/common/std/optional> in case of DUNE_HAVE_CXX_OPTIONAL is defined.

Motivation

If you have a compiler with c++17 std::optional support, the std implementation is imported into the namespace Dune::Std, but not all Helper classes are imported as well, i.e. nullopt_t, nullopt, in_place_t, in_place and bad_optional_access are missing. So, I neither can use e.g. std::nullopt nor can I use Dune::Std::nullopt, since the first one is defined whenever DUNE_HAVE_CXX_OPTIONAL is set and the second one if it is not set.

Discussion

For the future it has to be decided, whether std::in_place*** is implemented in <dune/common/std/optional> or better in <dune/common/std/utility>, since in the std library it is defined there and used not just for optional but also for variant and any.

changed the description

Any objections to merge this simple completion of the c++17-backport?

I'm not quite sure what anonymous namespaces are supposed to achieve (the const already gives nullopt and in_place internal linkage). Normally an anonymous namespace in a header is an error, though in this case it's hard to see how it'd lead to problems (you're not going to instantiate a template with a Dune::nullopt template argument). On the other hand, the existing compatibility implementation uses anonymous namespaces (equally needlessly), so there.

I guess it's gonna be gone anyway when we require C++17.

merged

mentioned in commit 76cf27a3