Adding a new low frequency feature
Adding a new function to compute the power ratio of a signal in the low freq range (0.02-0.5 Hz) and signal in the main band (2-4 Hz or 20-50 Hz) based on Lundstrom et al. 2021 https://www.medrxiv.org/content/10.1101/2021.06.04.21258382v1.full.pdf