#307 Emphasise DFT vs. FFT choice up front.

[Venks I]

The first line says this: "The math::fourier package implements two versions of discrete Fourier transforms, the ordinary transform and the fast Fourier transform.". That implies that both versions are available for use.

Reading top to bottom it sounds like there are only commands for dft (and none for fft), which is not true.

"The math::fourier package uses the fast Fourier transform, if applicable, or the ordinary transform to implement the
discrete fourier transform." might be more appropriate.