Option to skip installing Python dependencies in run scripts

[MakisH]

While automatically installing the dependencies for Python-based tutorials is convenient for some use cases (new user runs such a tutorial on their system), it is inconvenient when using a system already prepared. This includes:

1. training systems (USB/VM), where we provide a virtual environment already prepared, and which should work offline.
2. system tests, where we want to test specific versions (see also Propagating Python bindings version in system tests (conflict with requirements.txt) #584).

We could provide an option to these scripts to skip installing the dependencies (e.g., --skip-setup).

This would simplify a few aspects, and would be an easy solution for #584.

Also related to #547 and #526.

I want to work on this soon.

[fsimonis]

Does using --system-site-packages everywhere solve this? https://docs.python.org/3/library/venv.html#cmdoption-venv-system-site-packages

[fsimonis]

it is inconvenient when using a system already prepared

What you describe is a global flag. Would an environment variable be an easy way of handling this?
This would avoid having to handle parsing arguments, which can get nasty once there is a need for additional ones.

Also note that the venv ensures that there is a python3, something that distributions do not.

[MakisH]

Does using --system-site-packages everywhere solve this? https://docs.python.org/3/library/venv.html#cmdoption-venv-system-site-packages

No, because we typically don't want system packages leaking into the tutorial environment (e.g., system Nutils version with specific tutorial Nutils version).

Would an environment variable be an easy way of handling this?

Yes, this would be better. Continuing in #680.