VisualizeMultiShellScheme

% visualize single or multiple shell schemes
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% USAGE:
%   VisualizeMultiShellScheme(grad1)
%   VisualizeMultiShellScheme(grad1, grad2, grad3)
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% INPUT
%   varargin :  One or more matrix for schemes in different shells.
%               Each matrix has size N_ix3, N_i is the number of samples in the i-th shell.
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% Copyright (c) 2014, Jian Cheng <jian.cheng.1983@gmail.com>
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