Implementation Pressure entropy SPH

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.

See merge request !252