REF: ensure O(N log N) when using fft for acf

The numpy implementation will fall back to a O(N**2) version of the fft
in cases where it cannot factorize the array length, which can be a
increase in runtime of several order of magnitude. If we always ensure
to fill up the array with zeros that it's total length is a power of 2
the runtime will be guaranteed to be O(N log N).