Add Pressure-Energy (P-U) SPH

This MR adds Pressure Energy SPH, and makes one breaking change: renaming pressure-entropy SPH from hopkins to pressure-entropy. Users will need to configure --with-hydro=pressure-entropy rather than the --with-hydro=hopkins that they are used to.

Results from the hydro tests are already available on the wiki: https://gitlab.cosma.dur.ac.uk/swift/swiftsim/wikis/hydro-tests

It might be worth somebody having a look to see if I'm doing anything stupid performance wise, but from my perspective things have been running as fast as P-E.

Implements #197 (closed).

examples/KelvinHelmholtz_2D/kelvinHelmholtz.yml

@@ -8,8 +8,8 @@ InternalUnitSystem:
   
 # Parameters governing the time integration
 TimeIntegration:
-  time_begin: 0.    # The starting time of the simulation (in internal units).
-  time_end:   1.5   # The end time of the simulation (in internal units).
+  time_begin: 0.0    # The starting time of the simulation (in internal units).
+  time_end:   4.5   # The end time of the simulation (in internal units).
   dt_min:     1e-6  # The minimal time-step size of the simulation (in internal units).
   dt_max:     1e-2  # The maximal time-step size of the simulation (in internal units).
 
@@ -17,7 +17,7 @@ TimeIntegration:
 Snapshots:
   basename:            kelvinHelmholtz  # Common part of the name of output files
   time_first:          0.               # Time of the first output (in internal units)
-  delta_time:          0.25      # Time difference between consecutive outputs (in internal units)
+  delta_time:          0.01      # Time difference between consecutive outputs (in internal units)
 
 # Parameters governing the conserved quantities statistics
 Statistics: