More cosmology work

This is a large package of changes mostly affecting i/o of quantities for cosmological runs.

This implements #407 (closed) and #404 (closed).

In details, this adds:

• Cosmological evolution of the softening lengths.
• Creation of gas particles from the DM ones.
• Setting a minimal temperature for the gas.
• Re-scaling of the velocities read in for cosmology.
• Drifting of the velocities for the snapshots.
• Output of the potential in the snapshots.
• Changes the EAGLE_* example scripts to use cosmological time integration by default.

examples/EAGLE_100/eagle_100.yml

@@ -37,15 +37,18 @@ Statistics:
 
 # Parameters for the self-gravity scheme
 Gravity:
-  eta:                   0.025    # Constant dimensionless multiplier for time integration. 
-  epsilon:               0.001   # Softening length (in internal units).
-  theta:                 0.85      # Opening angle (Multipole acceptance criterion)
-  
+  eta:                    0.025    # Constant dimensionless multiplier for time integration. 
+  theta:                  0.85      # Opening angle (Multipole acceptance criterion)
+  comoving_softening:     0.0026994 # Comoving softening length (in internal units).
+  max_physical_softening: 0.0007    # Physical softening length (in internal units).
+
 # Parameters for the hydrodynamics scheme
 SPH:
   resolution_eta:        1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
   CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
+  minimal_temperature:   100      # (internal units)
 
 # Parameters related to the initial conditions
 InitialConditions:
   file_name:  ./EAGLE_ICs_100.hdf5     # The file to read
+  cleanup_h_factors: 1                 # Remove the h-factors inherited from Gadget