Chapter 8 of Statistical Rethinking 2nd Edition (#88)

* Added Chapter 6

* First attempt at Chp_07

* Data for Chp_07

* removed chapter 6 from 7 branch

* Refresh and Run All in botebook

* Progress towards #86. All but pairwise WAIC calculation done. Awaiting clarification from @aloctavodia.

* hacky solution to pairwise dse comparison

* run black_nbconvert

* clean start

* First draft of Chp_08

* ran through black_nbconvert

* restart and run all

* Data for Chp_08

* remove unneeded data

* Addressed @aloctovadia's comments (except standardised)