Stress warning for non-periodic structure

[frostedoyster]

See the failure on this test. The model predicts invalid values for the stresses on a water molecule, then the function to convert them to Voigt notation raises a warning.
https://github.com/lab-cosmo/upet/actions/runs/20901102264/job/60047076605?pr=67

[Luthaf]

I think this is fine. If the model is predicting a stress but gives nan/inf, the user should know about it