Make ppa work with PyMC

[aloctavodia]

Similar to #300 we should have a parser for PyMC

[nishant42491]

I'd like to try to fix this issue. A question I have is that in ppa and predictive_finder function signatures there's the parameter f_model. which is a Preliz Model class. What exactly is the Preliz Model class?

[aloctavodia]

It is a python function with preliz distribution generating random samples. It's explained here https://preliz.readthedocs.io/en/latest/examples/observed_space_examples.html

Why don't you keep working on the Dirichlet elicitation?