Multi-dimensional ellipsoid fourier filter.
The array is multiplied with the fourier transform of a ellipsoid of given sizes. 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. This function is implemented for arrays of rank 1, 2, or 3.