implement denormalize_pressure() for direct solver
make direct solver work with nonlinear schemes

We need to compute and return the correct initial_residual in
solve_stokes() when using a direct solver for the nonlinear methods like
iterated IMPES to work. This is some extra work, sadly.
add test
update changes.h
Merge pull request #132 from tjhei/direct_solver_with_nonlinear

fix direct solver with nonlinear schemes