Fast Python implementation of the Yin algorithm

Fast Python implementation of the Yin algorithm: a fundamental frequency estimator. Based on the article: \[1\] De Cheveigné, A., & Kawahara, H. (2002). YIN, a fundamental frequency estimator for speech and music. The Journal of the Acoustical Society of America, 111(4), 1917-1930. All the functions in the code correspond to steps in the article \[1\]. Meanwhile, the difference function has been modify substantially in order to improve speed. Finally, speed has been improved by more than 1000x