Compressible perplex

I added the option in the steinberger material model to use it in compressible cases. The incompressible case got a bugfix by this reformulation as well. Added a test for the compressible case that reproduces the simple compressible test (needed to change that test a bit to have a more reasonable initial condition). A test for the incompressible case is planned next, but I want to open a new pull request for that.