Refactor all power band functions

[jdpigeon]

1. Convert filterByRange to sliceFFT that slices an FFTBuffer based on an array of ranges given to it
2. Write averagePower operator that takes an FFTbuffer and returns the average power for each channel
3. Write alpha, beta, theta, etc. Power operators that perform sliceFFT and averagePower operators in sequence with predetermined ranges
4. Write powerByBand operators that returns an object with average band powers for all the classic EEG bands. Should also be able to take an object which defines names and ranges of custom bands
   ex: { delta: [c1, c2, c3, c4]. theta: [c1, c2, c3, c4], etc}