Entropy adiabat benchmark

This folder contains a technical benchmark for the entropy advection method. The model setup is described in entropy_adiabat.prm and requires the shared libraries in the plugins/ directory. In particular it requires a material model (plugins/entropy_model.cc/h) that is formulated using pressure and entropy as independent variables, a different assembler that assembles the entropy equation (plugins/entropy_advection.cc/h) and a different way to compute the adiabatic background profile (plugins/compute_entropy_profile.cc/h).