feat(cosmo): add MRT to I/O options - #18952

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Will try to have a look, but may have to be in a few weeks, as teaching is taking most of my time...

@mhvk thanks! No rush on this one. I don't think people are burning to have MRT support. It will be more useful when I finally figure out how to implement array-valued cosmologies.

I can have a look when it comes out of draft. 😄

Ping @taldcroft. Thanks! :)

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Ping @taldcroft for a review :)

@nstarman - sorry, I'll have a look at this.