Add an example of predicting on a new, unseen group in the existing notebook about hierarchical models

Proposal

In the existing notebook A Primer on Bayesian Methods for Multilevel Modeling, add an example at the bottom, in the section about prediction, about making predictions on a new, unseen group.

Why should this notebook be added to pymc-examples?

The prediction section of the tutorial on multilevel models says that one can make predictions on existing and new groups. But it only gives an example of predicting on an existing group. This addition would fill a gap noted in this Discourse thread.

References

Luciano's reply in the aforementioned Discourse thread has some clues for what the example might look like, though it needs to be updated from pymc3 to pymc.