Merge pull request #3 from jdannberg/grain_size_dependent_rheology

Grain size dependent rheology
Disabling full_pressure and use adiabatic pressure for viscosity.
Introduce minimum and maximum viscosity.
Add perplex and hefesto abilities to damage rheology.
Merge branch 'grain_size_dependent_rheology' of gmod10:~/office/Software/aspect into grain_size_dependent_rheology
add option to advect -log grain size