Allow C++17

I think quite some new features speak for allowing C++17. There are some compilers supporting all features, gcc 8 or gcc 7 (without std::filesystem), clang 8 or 5 (without std::filesystem). https://en.cppreference.com/w/cpp/compiler_support. However, there seems to be no cray compiler.

Who is using the newest Dumux version on a platform without C++17 support?