Implement autocorrelation

[clarkenj]

Correlating the signal with itself, with a lag. Indicates whether the pitch is repetitive. Nice little explanation here.

References (AD/MCI):

• Yancheva et al. (2015)
• López-de-Ipiña et al. (2013)

I'm struggling to find implementation code, however from my understanding the autocorrelation is used to estimate f0 so maybe it's part of that. I will keep digging.