3D convolution in the FFT domain

Achieve a 3D convolution in the fourrier domain.

%% convolution3D_FFTdomain - Performs a fast 3D convolution between a volume and a kernel using mutliplication in the Fourrier space.
%
%Syntax: [outVol] = convolutionInFFTdomain(inVol,inKer)
% Inputs:
% inVol - input volume (real / complex)
% inKer - input kernel (real / complex)
% Outputs:
% outVol - output convolved volume (real / complex) - precision of the output format is the same as the input
% volume. The output volume is the central part of the convolution with same size as inVol.
% size(outVol)=size(inVol) ('same' option of convn).

It is a specialized version of http://www.mathworks.com/matlabcentral/fileexchange/24504 to 3D convolution between volume and kernel (small).
Gives faster results for small volumes (<128x128x128, kernel 20x20x20), and of coure much faster than convn (x300).