[!342] Add a C++14 implementation of C++17's std::variant

Merge branch 'feature/stdvariant' into 'master'

ref:core/dune-common In dune-subgrid we wanted to use std::variant which is
part of C++17's STL ([see e.g. here]). Since the corresponding compilers (or
rather STL implementations) aren't too widespread by now and also out of pure
curiosity, I reimplemented std::variant myself using only C++14 and some of
Dune::Hybrid. In the end I ended up with a implementation that covers most of
the STL functionality and behaves more or less the same way (though it
probably won't be as safe, fast or complete as e.g. the gcc implementation, of
course). If this might be of any value for the Dune project, I'd be happy to
contribute this to dune-common.

See merge request [!342]

  [see e.g. here]: http://en.cppreference.com/w/cpp/utility/variant
  [!342]: gitlab.dune-project.org/core/dune-common/merge_requests/342