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.
Add test for internal heating statistics.
Minimum refinement function for cell vertices.

Cleanup

Update test results.
Bugfix.
Merge pull request #138 from gassmoeller/flexible-steinberger-viscosity-boundaries

Bugfix.
Entry in changes.h.
Another entry for changes.h
Another one.
Performance improvements.
add melt material model with latent heat

fix some bugs

indentation

clean up and add documentation

add some documentation

add two tests
Merge pull request #139 from gassmoeller/gplates-output

Clarify error message.
Update test results.