PitchYinFFT updates (new weighting parameter, do not throw exception on empty peak detection)

[xaviliz]

1) Add weighting parameter to select different weighting funcitons in PITCHYINFFT algorithm. The available choices are:

• the default weighting function
• A-weighting
• B-weighting
• C-weighting
• D-weighting

• Z-weighting.

  Note that there is no experimental validation of better pitch estimation performance with new alternative weightings.

2) Change behavior on empty spectrum peak detection: output zero pitch and confidence instead of throwing an exception.

@dbogdanov Edited for completeness.