Add DuneMatrix class.

This is a hack to get a more efficient constructor for dune-istl
matrices from Eigen matrices.
Compile fix for Dune 2.2.

Some BCRSMatrix members are new in 2.3.
Remove unused code.
Obtain dune-istl version for config.h.

Required by DuneMatrix.hpp.
Remove unneeded template argument.
Merge pull request #210 from atgeirr/dune-matrix-hack

Add a horrible hack: DuneMatrix class.