Add the code for Chapter 2 of Statistical Rethinking

[jvican]

I wrote the code manually to get more familiar with numpyro, but copied it over mostly from the numpyro port from the SSR book code:

https://github.com/fehiepsi/rethinking-numpyro/tree/master

One thing I take away from this experiment is how numpyro actually works at a high level.