Updating to add a trait class for darkmatter in cosmology. - #18760

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@nstarman If I am right this was a really small PR. I marked it as an internal class rather than a public one. Where do you want to impose checks such as say matter density = darkmatter + baryons and such ? I know its cosmology specific I was referring it to it in the context of FLRW.

So for turning these three ``` _BaryonComponent, _CriticalDensity, _MatterComponent," into public want one PR per component or a single PR ?

A single PR is good.

Thanks @preshanth !