[Dune] persistent containers

[Dedner, Andreas]

> On second thought, do I get the error above because I try to call
> operator[] on an Intersection, but Intersections are not Entities in
> general?

No there is no way. There is only the non-persistent index set available in PDELab or dnne-fem.
In many cases you should be able to 'fake' this by using the codim 1 version of the peristent container.
For conforming grids that works perfectly - for non-conforming grids you just have to find the
'correct' codim 1 entity to use, i.e., for ALU the codim 1 entity belonging to the (inside/outside)
entity of your intersection with the higher level.

Andreas