Mixing pool updates

[RomeshA]

Description

This PR adjusts mixing pools slightly to make them behave more interchangeably with networks. In particular, we encountered two issues

• Mixing pool infections were previously computed in the step() method, which runs prior to interventions unlike network-based transmission which occurs after interventions
• We had additional logic in our infect() method to adjust rel_sus and rel_trans prior to transmission, but this method was not used for mixing pool infections

These changes shift execution of mixing pool infections into Infection.infect() so that they occur in the same place and in a similar way to network based infections.

Checklist

• Code commented & docstrings added
• New tests were needed and have been added
• A new version number was needed & changelog has been updated
• A new PyPI version needs to be released