Update fitness and equilibrium support functions

[aornugent]

Addresses #354 #355 and #357.

Remaining tasks include:

• [ ] Rebuilding the slow_vignettes
• [x] Reviewing the Github Actions results

[aornugent]

Woo hoo! I think we're green.

I have not finished revisiting the vignettes, but hopefully this is enough for you to get to work on some gnarly science? 🤞🚀