Feature/interpolation

Interpolation between grids, initial version. Added traversal over irregular elements and gitlabci tests

dune/multimesh/mmiteratorbase.hh

@@ -6,6 +6,9 @@
 #include <numeric>
 #include <stack>
 #include <type_traits>
+#if DUNE_HAVE_CXX_VARIANT
+#include <variant>
+#endif
 
 #include <dune/common/std/type_traits.hh>
 #include <dune/grid/common/gridenums.hh>