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/common/CMakeLists.txt

+set(commonincludedir  ${CMAKE_INSTALL_INCLUDEDIR}/dune/subgrid/common)
+set(commoninclude_HEADERS  variant.hh)
+# include not needed for CMake
+# include $(top_srcdir)/am/global-rules
+install(FILES ${commoninclude_HEADERS} DESTINATION ${commonincludedir})