Add example showing how to access the underlying PyMC3 model

[GStechschulte]

This somewhat exists in the last section of getting started. Is this what this issue is referring to? Or is a more in depth example wanted; showing how to inspect, and modify the internally stored objects?

[tomicapretto]

Perhaps you can simply add another chunk showing how to fit the model as you would do with PyMC. Like this

with model.backend.model:
    idata = pm.sample()
idata