fix impedance freq indexing

[AurelienJaquier]

Description

We change the indexing of smooth_Z, but forget to also change the indexing of freq when applying a smooth_Z index. This should solve the impedance = 0.0 results that we were getting.

Checklist:

• Unit tests are added to cover the changes (skip if not applicable).
• The changes are mentioned in the documentation (skip if not applicable).
• CHANGELOG file is updated (skip if not applicable).