!4#2103 Rat functionality for `positivity`

Stream: PR reviews

[Heather Macbeth]

This PR by @Thomas Murrills has been awaiting review since Feb 6, and just got a merge conflict, I think from !4#2419, a to_additive fix by @Floris van Doorn. I don't understand it quite well enough to fix the conflict myself. Maybe someone who does could fix the conflict, and then review?

[Floris van Doorn]

I merged master and delegated the PR.