add example notebooks (and their execution through devops_tests)

[slayoo]

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[slayoo]

@zdaq12 this PR brings in examples from the PyPartMC-examples repo into PyPartMC with the aim of phasing out the -examples one. This is aimed at keeping things simple, at improving revision history consistency, and at streamlining development by ensuring that all new PartMC features will be checked against all example notebooks before mergning.

After this one is merged, I suggest to simply delete the PyPartMC repo.

From user perspective, little changes. The notebooks will not be part of the pip-installed package, and will remain executable on mybinder.org (incl. Voila) and Colab.