Changed behaviour of centroid_peaks when y1==y3.

Fixed issue #22 and maybe issue #20 (to be verified).
Sometimes, a centered peak was misplaced with an erroneous signal.
This was caused by applying the gauss fit to a section where there was no
peak.
This could add a few peaks when applying the centroid_peaks
function.

Signed-off-by: Pier-Luc Plante <pier-luc.plante.1@ulaval.ca>