Implement harmonics-to-noise ratio

[clarkenj]

See #5 for possible code. HNR used in Wiesman et al. (2023)

References (AD/MCI):

• Meilán et al. (2014): they look at the noise to harmonics ratio (NHR, inverse to HNR), found to be important for classification
• Mirheidari et al. (2019): HNR and NHR
• López-de-Ipiña et al. (2013): HNR and NHR
• Sumali et al. (2020): HNR

To decide: whether to include both. Highly anti-correlated but can give slightly different information