Harmonic Spectral Centroid
A variation on spectral centroid that is based upon peaks instead of bins.
Given a set of peaks, calculate the peak index that corresponds to 50% of
the energy in the window. Is based upon Stephan McAdams Spectral Centroid in
(McAdams 1999).
McAdams, S. 1999. Perspectives on the contribution of timbre to musical
structure. Computer Music Journal. 23(3):85102.
