cosmo: separate out tcmb - #17153

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Feature freeze is fast impending. I doubt this is going to make it to v7.0 but we can move the milestone later.

If #17151 gets in, this PR is also very uncontroversial. There's no change to any public API.

Thanks @eerovaher. Would you be willing to continue reviewing such non-API changing PRs? These are the preliminary steps towards #17150, but as you point out, are non-cosmological.

I don't know enough about cosmology to review the public API, but I suppose I can review refactoring implementation details.