[Dune] initializing parallel user data from file

[Dedner, Andreas]

> Two specific questions:
> line 279 in gridptr.hh: why is the partition Interior? I would expect InteriorBorder?
That would probably make sense but it seems only to be used for codim zero iteration and in
that case there should be no difference.
> line 286 in gridptr.hh: why swap and not an assignment? Is it a matter
of exceptions??
I guess I assumed it would be the most efficient - I did not want to copy the data. But
perhaps I'm wrong there...

Andreas