Updates to Simpon's Paradox notebook (pymc-devs#697)

* MutableData -> Data

* remove filter_vars="regex" from plot_trace

* update author edits section

* update watermark

* add random seed, model maths, use Gammas for sigma, simplify plot code

* add causal DAGs

* update summary section

* update admonition boxes

* update and simplify introduction

* update section headings

* simplify scatter plot code with seaborn

* code simplifications, model and idata renaming

* significant simplification of plot code

* update figure desccription

* Bunch of improvements and fixes

* Add Wilkinson notation for each model

* fix admonitions + add notebook reference to LKJ notebook

* final tweaks before review

* hide short plot code cell + grammatical fix

* update model 2 wilkinson notation

* add wikipedia link to DAG

* change equals to sim in model 3 maths

* partial progress on reformulating Model 3

* fixes

* model 3 fixes + further plot code simplification

* fix typo

* small tweaks and improvements

* remove commas from model 3 maths

* tweaks + abandon unobserved group distribution

* tweak plot sizes

* fix typo: "wayes" -> "ways"

* add clarification about _conditional_ posterior predictive

Author: drbenvincent