[poromechanics] pull fluid density out of volume variables

Merged Dennis Gläser requested to merge feature/poromech-pull-fluiddensity-out-of-volvars into master

For the coupled, problems this removes the dependency of the volume variables on the coupling context, which causes problems e.g. when using the standard vtk output module. We now obtain the density where needed in the local residual.

Edited by Dennis Gläser

Merge request reports