Fix complexity estimation of Fourier
Fix complexity estimation of Fourier

Previously, the Bluestein optimization were not regarded in the complexity
model, effectively causing some trouble. Also reduce code duplicates in
forward and backward by deriving the backward from the forward routine.

EMS-TU-Ilmenau/fastmat,EMS-TU-Ilmenau/fastmat,EMS-TU-Ilmenau/fastmat