HSGP: Birthdays Example (#627)

* init

* first basic version

* add some refs

* add some refs

* nicer syntax

* year to online references

* extra installs

* fix references

* typo

* fix metadata

* fix metadata

* add jupytext in metadata

* sklearn

* numpyro ref

* change zero sum normal to one hot encoder

* add comment first basis fuctions

* add bill as author

* feedback part1

* typo

* modularize via functions

* modularize via functions

* fix pre-commit

* hide some code

* rename obs -> time

* ZSN comment

* fix spacing

* fix class link

* empty commit

* rewrite intro

* final paragraph

* clean intro

* clean intro

* hide code

* add extension factor c to the intro as we are using it in the model

* feedback p3

* feedback

* correct complexity formula

Author: juanitorduz