Make FrameGenerator python3 compatible
Make FrameGenerator python3 compatible

Implement both __next__() and next() methods in FrameGenerator to support
both Python 2 and 3.

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