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.