Feature/interpolation

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

dune/multimesh/mmhierarchiciterator.hh

@@ -31,7 +31,8 @@ namespace Dune
 
     struct EntityStackEntry
     {
-      template <class Entity>
+      template <class Entity,
+        std::enable_if_t<std::is_same<std::decay_t<Entity>, HostEntity>::value, int> = 0>
       explicit EntityStackEntry (Entity&& entity)
         : it(entity.hbegin(entity.level()+1))
         , end(entity.hend(entity.level()+1))