Add chapter 11 of Rethinking 2 (#76)

* First commit

* Started work on Chapter 11

* Finished PP plot for chimpanzees

* Finished chimpanzees binomial section

* Finished aggregated chimpanzees

* Finished binomial regression section

* Finished prior pred plot for island data

* Started post pred checks islands data

* Finished geocentric islands model

* Wrote scientific islands model (fails)

* Finished Poisson offset section

* Finished multinomial section

* Finished chapter 11

* Ran black

* Improved scientific model of Oceanic tools

* Fixed typos

* Integrated Ravin's first comments

* Updated code 11.12 with transform arg

* Fixed Multinom model and added Osvaldo's comments

* Reran whole NB

Author: AlexAndorra