Merged requested to merge PressureEntropy_SPH into master
Here is a new flavour of SPH that works and gives better result than the default Gadget-2 scheme.
The only change affecting the rest of the code is the addition of the condition in
engine_init_particles() to create a second density loop over the particles for the special conversions that are required in this scheme. Other than that, it just fits in the rest of the framework.