Implementations for geometryInInside() and geometryInOutside()

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.

dune/subgrid/test/testvariant.cc

@@ -9,7 +9,7 @@
 #include <dune/common/exceptions.hh> // We use exceptions
 #include <dune/common/test/testsuite.hh>
 
-#include <dune/common/std/variant.hh>
+#include <dune/subgrid/common/variant.hh>
 
 // some non-default constructible type
 struct F {