Merge branch 'feature/matrix-free' into 'master'

Feature matrix free solution of linear PDEs

This implements matrix free solution of linear PDEs. Some notes:

1. 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.
1. 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.
1. 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 and I will update the issue accordingly.



See merge request !10