[Dune] ISI final suggestion:

[Christian Engwer]

Dear fellow developers,

>   after a further discussion of the IntersectionIterator issue in our
> local DUNE-meeting, we decided to propose the following solution for
> DUNE 1.0:
> 
> There should be all three intersectionIterators, i.e.
> IntersectionIterator, LevelIntersectionIterator,
> LeafIntersectionIterator, all satisfying the same interface.
> Thus the Entity interface provides the methods ibegin, iend,
> leafbegin, leafend, and levelbegin, levelend.
> ibegin and iend do the same as before and are not deprecated!

We also discussed this issue further and thought, that, if the
developers of the more complex grid consider the leaf/level version
easier, we should switch to this semantic in order to safe unnecessary
work.

I don't consider it a reasonable solution to introduce both
interfaces. We should definitely agree on one option and remove the
other. For 1.0 We need clear interfaces. We can tell the people this
interface is mature, this interface might change, but we should still
try to present dune with a consistent and clear interface -- people
will judge dune by the interface they see and by the time it takes
them to start playing with dune.

my 5 cents :-)
Christian