Fixed incorrect cosmology factors for pressure and internal energy in the i/o

Merged Josh Borrow requested to merge fix-internal-energy-cosmo into master

Previously we were writing that the internal energy transformed as a^{3(\gamma-1)}, which is incorrect. The pressures were also written to transform as a^{3\gamma}. In this MR I have rectified this to be correct, as


As we store P' and want the conversion factor to physical, we need it to be a^{-3\gamma}.

As for the internal energy, we can have a similar argument


where we need the factor for u to be a^{-3(\gamma - 1)}.

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