Comoving energy sph

Make the Minimal-SPH model use the correct comoving equations:

• Convert the physical internal energy into comoving one at start-up.
• Do not include the Hubble flow term in the expression for du/dt, only include it in the visosity tensor.

Also, print the value of the viscosity constant to the snapshot and report it in the Zel'dovich analysis plot since it plays a huge role.

No rush. I just opened this so that we do not forget about it.

merged

mentioned in commit cb315db3

mentioned in issue #401 (closed)