[dune-pdelab] Backend interfaces

[Christian Engwer]

> The vectors were requested quite a lot here in Heidelberg as an easy way to traverse
> the flattened set of DOFs contained in a vector. In particular, they allow you to simply
> iterate over single DOFs even if the actual vector is blocked in some way (maybe even
> several levels deep).
> 
> They are really useful for printing out / reading in vectors and scalar operations on the
> individual entries.

OK, so I'll use the raw pointers as iterators.

In general I'd think we should add a clear interface description for
the backends. Perhaps we could even re-use tricks learned in
dune-functions and use an abstract base-class?!...

Ciao
Christian