Add size bias

[swo]

• Put in a nascent chapter on generating functions
• Find some more compelling examples: What's a non-urn, non-infectious disease example?
• https://en.wikipedia.org/wiki/Robbins_lemma
• Include the CF approach to the generation interval problem: https://projecteuclid.org/journals/probability-surveys/volume-16/issue-none/Size-bias-for-one-and-all/10.1214/13-PS221.full