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