BDA Python demos spine

This section incorporates Aki Vehtari’s BDA_py_demos, a Python demo collection for Bayesian Data Analysis, 3rd ed.

The widened project scope is now Applied Bayesian Computing in Python:

The notebooks below are included as source material first; as the book matures, each will get an accompanying translation/commentary page that connects it to the ROS examples and the CmdStanPy/BlackJAX/Oryx patterns.