Use nbconvert to test notebooks in the GitHub CI

[waltsims]

nbconvert would allow auto-running notebooks and using output as readme for example
See blog post:
https://innerjoin.bit.io/automate-jupyter-notebooks-on-github-9d988ecf96a6

[waltsims]

The concept is to run scheduled examples on CPU on the main branch every night. Example notebooks would be converted to python files with nbconvert before execution. Since the runner does not have a GPU, the fallback should be CPU execution. These would only run if the head commit has changed.