Impedance should return an array, not a float (#403)

Co-authored-by: Jaquier Aurélien Tristan <[email protected]>

efel/pyfeatures/pyfeatures.py

@@ -171,7 +171,7 @@ def impedance():
  )[0]
  smooth_Z = gaussian_filter1d(norm_Z[select_idxs], 10)
  ind_max = np.argmax(smooth_Z)
- return freq[ind_max]
+ return np.array([freq[ind_max]])
  else:
  return None
  else: