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.

doc/RTD/source/HydroSchemes/sphenix_sph.rst

@@ -67,3 +67,14 @@ There is also a compile-time parameter, ``viscosity_beta`` that we set to
 ``hydro_props_default_viscosity_alpha_feedback_reset = 2.0`` and the
 diffusion is set to ``hydro_props_default_diffusion_alpha_feedback_reset =
 0.0``. These can be changed in ``src/hydro/SPHENIX/hydro_parameters.h``.
+
+Pressure Floor
+~~~~~~~~~~~~~~
+
+The pressure floor is implemented for this scheme. The 
+pressure floor is used in the sound-speed and hence the
+time-step will be lower in this case than if the pressure
+floor was not taken into account. The only other impact
+of the pressure floor is in the force loop, where the
+sub-grid floor pressure is used instead of the pressure
+calculated from the internal energy and density.