Implement pitch features

[clarkenj]

See #5 for possible code.

Calculate: pitch, mean, max and min, standard deviation.

Wiesman et al. (2023) implement "standard deviation of pitch measured over voiced segments (f0_std)". Note, F0 (the fundamental frequency) is the quantifiable acoustic property which humans perceive as pitch. The "pitch" that Praat returns is actually the F0.

References (AD/MCI):

• López-de-Ipiña et al. (2013): mean and std
• Yancheva et al. (2015): mean and variance of F0
• Sumali et al. (2020)

[clarkenj]

Also a nice example with parselmouth here.