Matter density as a trait - #18307

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@nstarman would you want me to move dark matter density and other related traits into this single class. I am happy to do that. I figured matter density was a good one to tackle before touching the energy densities.

I was wondering what you thought about having all the matter densities in one class. Include the dark matter here too.

I ask because I can compute dark matter density as matter - baryons and I can do that inside the MatterDensity class rather than have a separate one. I have nominally made a separate branch and moved the dark matter there too. I have moved the functions. If we agree on what formats. I am happy to do photon and dark energy along with distance measures next. I will leave the neutrinos to you.

I was wondering what you thought about having all the matter densities in one class. Include the dark matter here too.

I think it's best to keep them separate. We want to use these traits to build e.g. MatterOnlyCosmology, for which we should have only $\Omega_{m,0}$, $\Omega_{m}(z)$.