GetGradAllShells
================

.. code-block:: none

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