Unify all FFT interfaces and FFT based operators

As more and more FFT-based operators are implemented, it would be nice if we unified all different FFT interfaces:

• pyFFTW based on FFTW
• scipy.fftpack based on Fortran 77 library FFTPACK
• numpy.fft based on a C library forked from FFTPACK
• gpyFFT based on clFFT which is an OpenCL FFT library

This would greatly reduce the development time of such operators and adding a new FFT backend would be straightforward.