Fixes to the cosmo hydro

A bunch of all-around fixes that allow SWIFT to match exactly Gadget-2 on the SamllCosmoVolume hydro test.

This includes:

• Restore the testSymmetry to its old glory.
• Use the correct power of the scale-factor in the term designed to prevent numerical crashes in the Balsara switch.
• Add a Hubble flow term to the Balsara switch in the Minimal & Gadget schemes.
• Add the Hubble flow term to the viscosity SPH equation for the thermodynamic variable in the Gadget and Minimal schemes.
• Document properly that the minimal energy stored in the hydro properties is in the physical frame.
• Use the z=0 critical density when deciding the displacement of particles in space_generate_gas().
• Correct a sign mistake in the displacement of generated gas particles.