Update Posterior Predictive Checks Notebook (#3857)

* Changed sampling defaults and cleaned typos

* Finished updating PPC section

* Finished updating prediction section

* Ran whole NB

* Ran Black

* Revert "Changed sampling defaults and cleaned typos"

This reverts commit 5c08d92.

Revert changes made in sampling file

* Updated posterior pred NB

Author: AlexAndorra