PeakPickingProcessor skips peaks while combining them

[superbock]

E.g., if frames 2, 4, 6 are detections and combination width is set to be 3 (i.e. do not two consecutive onsets closer than 3 frames), the detection at frame 6 will be skipped because it is within 2 frames to the predecessor -- but it should not, since the predecessor was removed. We need to loop over all detections to handle this correctly.