Sphenix pressure floor

In this merge request, I am implementing the pressure floor for SPHENIX. As it is a density scheme, the solution is straight forward and simply requires to use the pressure with the floor in both the momentum and internal equations.

This was tested with the new example PressureFloor. It is simply the cooling box with self gravity. The setup is done in order to collapse quickly (about 1min of simulation) without pressure floor. With the SPHENIX pressure floor, I am able to avoid this collapse.

@jborrow, I am touching a bit SPHENIX, therefore I guess that you may want to take a quick look.