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.
Bugfix.
Add tests for both viscosity calculations.
Clarified test description.
Merge pull request #137 from gassmoeller/flexible-steinberger-viscosity-boundaries

Flexible steinberger viscosity boundaries
Clarify error message.
Internal heating statistics computes mass averages.

Internal heating statistics should compute mass averages and integrate
over mass instead of volume since the internal heating rate is also
given as specific heating rate (W/kg instead of W/m^3). In the non-
dimensional testcases this was not a problem so far, because the
density was commonly assumed to be 1 and constant.