Add testing of tutorial notebooks using nbmake

[egpbos]

Fixes #89.

[cwmeijer]

As it's still failing tests and there is no recent activity, we don't think you want us to review it, so I changed it to draft. ♥

[sonarqubecloud]

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[egpbos]

Ok, reviewers, this PR now does three four things:

1. Add nbmake jobs, which, obviously, fail currently. I opted not to fix them here, but just let them fail, since people are still working on the notebooks in other PRs as well. This is also the reason I made them separate jobs, instead of including them in the build jobs; build then still passes, as it should.
2. Use a new Actions feature called Composite Actions to remove duplication from the jobs. I added a custom Action that installs Python, DIANNA and dependencies. This action is then referenced from all the other jobs.
3. The new Action also has an added feature: it caches pip. This could be faster than reinstalling every time. It could also be slower :D This is just an experiment.
4. Renamed the build job to just "build" (see Badges & additional readme paragraphs #52).

[cwmeijer]

Tested nbmake command locally. looks ok. Great addition.
Refactoring makes sense 👍
Thanks for the renaming of 'build' :-)
lgtm+