Feature matrix free solution of linear PDEs
This implements matrix free solution of linear PDEs. Some notes:
- Right now the code simply uses BiCGSTAB from istl from istl for solving the linear systems. A more subtle solver setup should be postponed until the istl-ini factory arrived at PDELab.
- Poisson DG doesn't converge. Tests with the convectiondiffusiondg lop from PDELab show that the hand written lop has the same behaviour. Because of that I turned off DG tests.
- Right now nonlinear PDE's are not covered since PDELab doesn't seem to support nonlinear matrix free solutions. We probably have to adjust Newton first.
This merge request is related to #26 (closed) and I will update the issue accordingly.