src/mathutils.{c,h}: add fvec_quadratic_peak_pos, a fixed replacement for fvec_quadint
src/pitch/pitchyin.c: use fvec_quadratic_peak_pos
src/pitch/pitchmcomb.c: use fvec_quadratic_peak_pos
src/pitch/pitchyinfft.c: use fvec_quadratic_peak_pos
src/io/source_sndfile.c: mono to stereo, copying last channel of file to all remaining output matrix channels
src/io/sink_apple_audio.c: warn before fixing size
src/mathutils.h: markdown syntax