Update FrameGenerator Python wrapper
Update FrameGenerator Python wrapper

- Update documentation for FrameGenerator (#424)
- Inherit parameters defaults from FrameCutter instead of using
  hardcoded values for better consistency
- New method frame_times() to compute frame positions in time

carthach/essentia,MTG/essentia,carthach/essentia,MTG/essentia,MTG/essentia,carthach/essentia,MTG/essentia,MTG/essentia,carthach/essentia,carthach/essentia