[dune-pdelab] Backend interfaces

[Christian Engwer]

OK, I have now created two branches, feature/simplebackend and
feature/eigenbackend

The first one renames the dense backend and adds support for "simple"
CSR matrices. The second reimplements the eigen backend and ports the
solvers to the new interface. The old immplementations of matrix and
vector interface are abandoned as the changes were just too big.

Christian