[Dune] Vector of IntersectionIterators

[Andreas Dedner]

Intersections and Entities in ALU are stacked for reuse - so they
are freed only at the end of the program - not really a bug but
perhaps not the best solution (the assumption was that only
very few instances of entities or intersections would be used at
a given time in the program).
Andreas

> Destroying an IntersectionIterator definitely does not free it's
> space. It looks like a bug.
> 
> Dragan
>