[Dune] Storing entity iterators

[Christian Engwer]

Hi Marco, hi Steffen,

> You should not store entity iterators, that can be very expensive. With the copyable entities, you can simply store entities. But in your use case, you should probably
> use an EntitySeed. An EntitySeed is a compact representation of an entity, from which the Grid can reconstruct the entity without iterating over the grid. It works like this:

this is only partially true. It depends on your requirements. You have
tradeoff...
- EntityPointers (or in future entities) are heavy objects, but storing
  them is performance optimal, as you don't have to recreate them
- EntitySeeds store only the minimal data, but this requires
  recomputions to obtain the full entity.

Ciao
Christian