Impedance should return an array, not a float

[AurelienJaquier]

Description

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).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.24%. Comparing base (2cddd26) to head (79fb005).
Report is 24 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #403      +/-   ##
==========================================
+ Coverage   91.73%   92.24%   +0.51%     
==========================================
  Files          36       39       +3     
  Lines        6278     7389    +1111     
  Branches     2033     2299     +266     
==========================================
+ Hits         5759     6816    +1057     
- Misses        266      289      +23     
- Partials      253      284      +31     

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