Define Jupyter notebooks automatic tests

[Sinclert]

This issue describes the need to define a set of tests that run the existing Jupyter notebooks examples to check that changes in the codebase does not leave the examples broken.

These tests can be define by using Papermill, a Python package to execute jupyter notebooks, following what other Python libraries, such as pyhf, has done.

List of goals:

• Migrate current notebooks to Python3 syntax.
• Define set of tests using Papermill
• Include these tests on GitHub Actions (Same trigger conditions as the current CI workflow? different? 🤷🏻‍♂️ )

[Sinclert]

Notebooks migration PR: #480

[Sinclert]

Working-in-progress branch with Papermill tests: https://github.com/diana-hep/madminer/tree/test-jupyter-notebooks