[dune-pdelab] dense backend

[Christian Engwer]

Dear all,

I am writing a simple CSR matrix backend. This should work with any
simple container in the old fashioned CSR format. The matrix format
should be compatible with the dense vaector backend.

This brings me to the actual questions. I'd like to rename the dense
backend stuff to simple, as this is the actual idea here. We can then
have a SimpleVectorBackend, SimpleDenseMatrixBackend and a
SimpleSparseMatrixBackend. Any objections?

Ciao
Christian