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
fix embarrassing compilation error
fix denormalize pressure in parallel

We were not handing a ghosted vector to denormalize pressure when using
a direct solver, which caused a runtime error.
Merge pull request #133 from tjhei/direct_solver_fixes

fix denormalize pressure in parallel
add documentation for melt fraction postprocessor
Merge pull request #134 from jdannberg/melt_documentation

Melt documentation
Added a constant scale factor to the GPlates plugin.
Use pcout for screen output.
Fix zero velocity handling.
Do not modify pcout for GPlates output.
Improve documentation.
Simplify logic and fix output bug for 3d.
Merge pull request #135 from gassmoeller/gplates-velocity-fix

Gplates velocity fix
add gravity visualization
Merge pull request #136 from tjhei/gravity_vis

add gravity visualization
Made steinberger material viscosity calculation more flexible.