Use Dune::is_detected

fixes #602 (closed)

Dune2.6 already provides a fallback implementation of std::is_detected (C++17). We can use this to replace the rather lengthy and complicated isValid() method.

It is substantially less typing and easier to read. However, you need two helper structs instead of one (but each one being a one-liner).

Also seems to work with gcc5.5