Implement read-only IdentityMatrix

This implementation of an IdentityMatrix does not hold any data. It
implements a reduced version of the DenseMatrix interface including a
standard C++ cast operator to a Dune::FieldMatrix. This code illustrates
the new forward capabilities of DenseMatrixAssigner.
(cherry picked from commit 64df6274)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>