Change peakpicker to match API specs, make quadint per channel

Change peakpicker to match API specs, make quadint per channel
 * src/mathutils.c
   - add per channel mean and median
   - update moving thres and adapt_thres accordingly
   - change quadint unused span argument to a channel argument
 * src/onset/onset.c:
   - make wasonset a vector for multi channel, use new peakpicker
 * src/onset/peakpick.c:
   - update peakpicker do for multi channeling
 * src/pitch/: update use to fvec_quadint
 * src/tempo/beattracking.c: update calls to fvec_quadint
 * src/tempo/tempo.c: update peakpicker usage
 * tests/src/test-peakpick.c: update peakpicker usage