Timeseries models derived from generative graph (#642)

* nb init

* intro

* first interation

* fix code references

* feedback part 1

* fix pre-commit

* feeedpack part 2

* feedback part 3

* conditonal posteror

* forecast

* feedback part 2.1

* typos

* rename time series length

* cleanup

* update pymc

Author: juanitorduz