Clean-up assign-example-ids

[MatthiasWiesmann]

The assign-example-ids.py script was stand-alone python script that was execed from the main script.

• Move the code into an actual function.
• Instead of going all the examples twice, keep track of the changed examples and their path in a dictionary.
• Perform atomic rewrite of example files.
• Renamed to assign_example_ids so it can be a package.
• Call this package's function from the main script.
• Use logging statements consistently.