[ImgBot] Optimize images

*Total -- 1,948.33kb -> 1,580.64kb (18.87%)

/images/ch5-risk-outcome.png -- 154.69kb -> 62.68kb (59.48%)
/images/Lecture-1_SIR-Models.png -- 118.04kb -> 69.36kb (41.24%)
/images/ch5-fear.png -- 50.01kb -> 40.02kb (19.97%)
/images/iris.png -- 1,591.70kb -> 1,375.25kb (13.6%)
/images/06_mermaid.png -- 33.90kb -> 33.33kb (1.67%)

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Author: ImgBotApp