[dune-pdelab] Backend interfaces

[Christian Engwer]

Dear all,

I'm again working on backends... this time I tried to reactive the
Eigen backend. In the last revision of the Backend interfaces the
vector got iterators. Steffen could you please elaborate on these? Are
they really required? The thing is that Eigen does not actually
provide Iterators. We can use raw pointers, but in any case I'm
wondering why we need the iterators.

Ciao
Christian