Faster 3D Walsh - Hadamard Transform (sequency, natural)

Implements a 3D Walsh Hadamard transform. This function is FAST!

Copy&Paste from the file:
%WAT3D 3D Walsh or Hadamard Transform
% WAT3D(X, order) will transform the 3D matrix X to the Walsh or Hadmard
% domain depending on the parameter 'order'.
% If order == 'sequency' or 'walsh'
% The function will transform the matrix using the sequency ordered
% walsh functions.
% If order == 'hadamard' or 'natural'
% The function will transform the matrix using the natural ordered
% walsh functions. This is also known as the hadamard transform.
% TODO: order == 'dyadic' is not yet implemented.
% If no order is set the default 'sequency' is assumed.
%
% This function handles only the cases where N, N/12 or N/20
% is a power of 2 if order == 'natural' or 'hadamard'.
% If order == 'sequency' N must be strictly a power of 2.

% Reference:
% http://mathworld.wolfram.com/WalshFunction.html
% http://ieeexplore.ieee.org/iel5/12/35157/01674860.pdf
% http://dx.doi.org/ --> doi:10.1016/j.physletb.2003.10.071

This implementation is fast! I use it to call it many thousand times. It is way faster than the internal function (fwht).