Multiple EOS materials

For planetary SPH simulations (e.g. giant impacts) we need to allow different SPH particles to have different equations of state. They still have all the same interactions just like gas particles now, it's only the EOS calculations that change.

Current plan is to add an integer material flag property to all particles, which can default to e.g. 0 and is ignored in all usual cases. This flag will be an additional argument for all EOS functions (along with e.g. density and entropy) and case switches inside can choose between the different materials.