This implements said methods geometryInInside() and geometryInOutside() for SubgridLeafIntersections. While the concept of creating new geometries via Dune::MultilinearGeometry where needed was rather straight-forward, the actual implementation took quite a bit of work due to the different types. @akbib and @maxka came up with using C++17's std::variant and @akbib did the work implementing this. I contributed a fallback implementation of std::variant that does not need C++17.