Santa barbara cluster and correct cosmological terms in Gadget-SPH

• Remove a spurious a^2 term in the entropy evolution in Gadget-SPH.
• Do not include the Hubble-flow term in the SPH equations. Only keep it in the viscosity tensor.
• Add the Santa-Barbara cluster comparison example.

@jwillis could you cross-check that the vectorized and non-vectorized SPH interaction routines match in the Gadget2-SPH case: src/hydro/Gadget2/hydro_iact.h ? Thanks!

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• 5571ddfc - Added missing factor of r2 to dvdr_Hubble.

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All that was missing was a factor of r2 to dvdr_Hubble. Other than that it seems fine to me.

Thanks!

merged

mentioned in commit a58d1c60