GetGradAllShells

% Get Nx3 gradient table from gradients in all shells.
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% USAGE:
%    gradAll = GetGradAllShells(gradCell)
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% INPUT
%    gradCell  :  a cell array with M gradient matrix from M shells.
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%
% OUTPUT
%    gradAll   :  Nx3 gradient matrix from all M shells.
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% Copyright (c) 2014, Jian Cheng <jian.cheng.1983@gmail.com>
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