Fix compatibility to numpy without rfftfreq.

Fixes #599.
`np.fft.rfftfreq` should be faster and it ensures that the returned frequencies match the `rfftfreq` implementation of numpy (if that for some reason were to change).
Either way is ok with me, but if we go only with our implementation I might spend some more time on it to make it a bit faster (the `np.abs` is only needed for the last element). I am not sure how the speed would be a concern.