This changes the interface of the FFTOperator according to today's discussions.
Documentation was added; tests have been adjusted.
NOTE: after this change, the Wiener filter demo no longer works, since it calls "inverse_times" for an FFTOperator.
Also, the SmoothingOperator no longer works, since it also requires the inverse FFT to work. I expect that it should not be allowed to work on spherical spaces anyway. Let's discuss this tomorrow.