Multi-dimensional Gaussian fourier filter.
The array is multiplied with the fourier transform of a Gaussian kernel. If the parameter n is negative, then the input is assumed to be the result of a complex fft. If n is larger or equal to zero, the input is assumed to be the result of a real fft, and n gives the length of the of the array before transformation along the the real transform direction. The axis of the real transform is given by the axis parameter.