Blackbox likelihood

[OriolAbril]

file: https://github.com/pymc-devs/pymc-examples/blob/main/examples/case_studies/blackbox_external_likelihood.ipynb

Required changes

The notebook needs to be modified to use ArviZ+InferenceData at the very least, it's hard to know what extra work will be needed as it has not been executed for a while, cython usage is tricky to get right. Note that it uses DensityDist is strange ways, using dicts as observed that contain freeRVs and will need to use density_dist_obs=False as idata_kwargs (ref)

Changes to discuss

I don't think this notebook can be converted to v4 unless it undergoes a significant rewrite. Not sure if we should try to get it working with a custom distribution or with pm.Potential for v4.

⚠️ It requires cython to run.