Use Jupytext notebooks

The idea is to store notebooks as markdown files, so that under Git it's easier to understand the changes.

It's the very first time I'm using Jupytext, and I don't guarantee I did all correctly:

• I converted all notebooks with jupytext --to md, and then removed the original ipynb files.
• I added Juytext in the conda environment.
• I added a jupytext-configfile to ensure the new Markdown files are directly interpreted as notebooks (I don't use the "pairing" possibility that keeps a ipynb file along the Markdown file

Feel free to tell me if it can be done in a better way!

A known limitation in current configuration is that with Binder the Markdown files are not loaded by default as notebooks (but with 'open with' it works...)