PyMC implementation of M0 model

[fonnesbeck]

No description provided.

[JTorgander]

Thank you for contributing! I have made the following edits:

1. Adding a marginalised version of the model which now is equivalent to the corresponding Stan model
2. Renaming your submitted model as "M0_model_discrete" for clarity
3. Created a separate PR for the reference draws (we haven't yet decided on a submission format for reference draws generated outside Stan).

[MansMeg]

• Jakob check why tests fails