Add missing requirements.txt to oscillator and oscillator-overlap tutorial

[NiklasVin]

This PR adds the requirements.txt files for both python solvers of this tutorial, that were missing.

[MakisH]

Thank you for spotting this!
I think this case is a bit special, since we have the solver in a dedicated directory.

There might be more similar cases to update accordingly.

[MakisH]

In this case, we have the solver in ../solver-python.
Would it make sense to create the requirements.txt and the venv there, to avoid duplication?

[BenjaminRodenberg]

I think that's a good point that could also save us a lot of implementation & maintainment work in #547.

Let's do the following here: Let's get this PR merged with the structure as it is. Moving requirements.txt and the venv into the solver is a general thing that we should discuss in #574 and then solve consistently (maybe even for the nutils cases). I'll add a comment there.

[BenjaminRodenberg]

Maybe consider adding oscillator-overlap in this PR as well? I think everything should be pretty identical to oscillator.

We could also do this in a new PR, but if @NiklasVin is doing another round of testing before merging then I think doing oscillator-overlap in the same sweep would be a good idea.

Took care of this since I was running the case anyways. Will merge this PR now.

[BenjaminRodenberg]

I did a quick test run of this case. After adding scipy everything works.