Tuned processing notebook

Not many changes. I tried to structure a bit more the notebook to have a clear view of the steps I need to show during the live coding.

The notebook shows how to:

• use shell commands in a cell (ls, ln -s to reach /data)
• use jupyterlab-h5web in a cell
• write and execute Python code (get data out of a HDF5 file, write the PONI in a file)
• display images and curves with matplotlib (frame and integrated result)
• write a bit of Markdown (title, lists, links)

Tell me if you think some other steps should be shown