Functions for convolution See: https://en.wikipedia.org/wiki/Convolution
convolve : List Basics.Float -> List Basics.Float -> List Basics.Float
Convolve two real number lists, using FFT for lists longer than 128 elements or direct convolution otherwise. Threshold was determined by benchmarks.
convolve [ 1, 0, 0, 0 ] [ 0, 0, 1, 1 ] --> [0, 0, 1, 1]
convolveDirectly : List Basics.Float -> List Basics.Float -> List Basics.Float
Convolve two lists of numbers directly by $$ zs_j = \sum_i^n {xs_i * ys_{j - i}} $$