Feature/improve variant
- Implement and use a cmake check instead of
__has_include(<variant>)becauseg++-7 -std=c++14defines the header but notstd::variant - Pull
std::monostateinto the Dune::Std:: namespace and provide a fallback implementation.
__has_include(<variant>)
because g++-7 -std=c++14 defines the header but not std::variant
std::monostate into the Dune::Std:: namespace
and provide a fallback implementation.