Python-based tutorials: Log list of installed packages

[MakisH]

In the run.sh of our Python-based tutorial, we create a venv and install packages. This PR adds a call to pip freeze that writes the list of installed packages (with their versions) into a file called pip-installed-packages.log. This additional information can increase reproducibility.

The log file is not touched by clean.sh, as the respective .venv is also not removed by it. It is also neither a case nor a preCICE log file, to be added to the respective cleaning functions.

This is related to #596.

Checklist:

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[uekerman]

Makes sense to me. Whether or not to clean is probably debatable, but I understand your reasoning.
I did not try it and did not check whether you caught everything.