Non-deterministic output of the _diffusion_sin_1D.py

The _diffusion_sin_1D.py test fails from time to time and causes the pipeline to fail sometimes. My main suspicion is that it is caused by an OpenMP mistake, introduced by some commit.

I would recommend going back in the commits until the mistake disappears and identify the modification responsible for this.

EDIT: The problem comes indeed from an OpenMP mistake, which causes a small numerical error that grows with the number of threads. Sadly, I don't have the time to fix it on my own.