Improve notebook retaining wall

[kawojciechowska]

Probabilistic library version checks

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• I have checked that this bug exists on the latest version of Probabilistic library here.

Kind of issue

Documentation is missing

Location of the documentation

https://github.com/Deltares/ProbabilisticLibrary/blob/master/web/notebooks/Case_1_retaining_wall.ipynb

Documentation problem

Convergence plots are empty in the notebook.

Suggested fix

add project.settings.save_convergence = True