Fix code in exercises

[lagru]

Addresses a few errors when executing the notebooks. I also took the liberty to use a more sensible and consistent numbering of parts and files.

[lagru]

Ready to merge. The reported failures when building

tutorial/03_segmentation_and_measuring.md: WARNING: Executing notebook failed: CellExecutionError [mystnb.exec]
tutorial/03_segmentation_and_measuring.md: WARNING: Notebook exception traceback saved in: tutorial/_build/html/reports/03_segmentation_and_measuring.err.log [mystnb.exec]
tutorial/03_segmentation_and_measuring.md:23: ERROR: Document orsection may not begin with a transition

are due to missing points that are intended to be picked by hand.

[JoOkuma]

today is a holiday here, I'll take a look at it tomorrow, @lagru

[stefanv]

Swap Lars and Stéfan, so Lars appears first.

[lagru]

I thought to list them in speaking order. But I don't mind switching them around.

[JoOkuma]

@lagru, the jupyters worked and everything seems correct, except it took me a minute or two to be able to load the .md as a jupyter notebook.
I didn't know how that worked and I had to right click it and select the Notebook option. Otherwise it was loaded as plain markdown.

It may be worth updating the "Running the notebooks" section on install_dependencies.md

[lagru]

Thanks @JoOkuma. That's a good point about needing to right-click in jupyter lab. In think that's a limitation of jupytext. Jupyter notebook opens these fine. Would you like to add a comment? :)