[Dune] dense matrices in dune

[Christian Engwer]

Hi Franz,

> I noticed that there is also a dense matrix class in dune-istl.
> Is it possible to use all the iterative solvers and functions also
> with this type of matrix? (efficiency? LU-decomposition?
> DenseMatrix-SparseMatrix-multiplication? ...)

this is the goal. atm it isn't possible yet, as some methods aren't
implemented yet. The problem is that in general the return types
aren't clear. I intend to add type promotion. Then it should work with
all lin-alg.

Christian