ENH: Time evolution Schechter amplitude

[Lucia-Fonseca]

Description

This PR adds a function that returns the amplitude of the Schechter function for active galaxies based on equation (17) in de la Bella et al. 2021. Merging this PR closes #515.

Checklist

• [x] Follow the Contributor Guidelines
• [x] Write unit tests
• [x] Write documentation strings
• [x] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request

[Lucia-Fonseca]

why is it not passing all of the checks?

[rrjbca]

Check the logs

>       assert output_today == phi_today
E       assert 0.0037757219092541604 == 0.00377572190925416

Given the relative numerical complexity of the function it seems reasonable that the results are not identical but agree to some high precision.