fix: spectrum smoothing and autocorrelations

[mdenolle]

• smooth_N was used for spectral smoothing, it should be smoothspec_N
• DC component should not be zero for autocorrelations

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.52%. Comparing base (337c737) to head (3e277a4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #315   +/-   ##
=======================================
  Coverage   83.52%   83.52%           
=======================================
  Files           9        9           
  Lines        1973     1973           
  Branches      332      332           
=======================================
  Hits         1648     1648           
  Misses        207      207           
  Partials      118      118           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.