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.

examples/SedovBlast_1D/sedov.yml

@@ -21,7 +21,7 @@ Snapshots:
 
 # Parameters governing the conserved quantities statistics
 Statistics:
-  delta_time:          1e-3 # Time between statistics output
+  delta_time:          1e-5 # Time between statistics output
 
 # Parameters for the hydrodynamics scheme
 SPH: