Add test checking if all notebooks execute without an error

[Patrick Schmitt]

I just came across how to test jupyter notebooks https://www.blog.pythonlibrary.org/2018/10/16/testing-jupyter-notebooks/.

Maybe adding a simple test just checking if all tutorials pass without an error is a good idea?
But maybe it is overkill...

[Fabien Maussion]

The book build allows errors on build:

execute:
  execute_notebooks: auto
  timeout: -1
  allow_errors: true

We could set this to false and see what happens...