Adding tests for ppa

[rohanbabbar04]

Solves #131

Description

Added tests for ppa

• Added test_ppa.py
• Added ppa.ipynb

Checklist

• Code style is correct (follows pylint and black guidelines)
• Includes new or updated tests to cover the new feature
• New features are properly documented (with an example if appropriate)
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• Widget states have been properly saved (only for notebooks with widgets) see for details.

[rohanbabbar04]

@aloctavodia
I am getting errors when setting engine=bambi, is it because it is currently not fully implemented or I am doing something wrong..

pz.ppa(a_preliz_model,engine="bambi")

Ran into some errors while applying the method in the above manner.

[aloctavodia]

The parser for Bambi is very brittle at this point. maybe better to skip it. But we should open and issue to don't forget to add it later.

[rohanbabbar04]

Sure, Currently I have not added it
I will let you review this PR, then will open an issue for engine=bambi

[aloctavodia]

LGTM!