WriteDirections
===============

.. code-block:: none

  % Write gradient table to a file
  %
  % USAGE:
  %   WriteDirections(grad, gradStr)
  %
  % INPUT:
  %   grad     : Nx3 matrix
  %   gradStr  : output file name
  %
  % Copyright (c) 2014, Jian Cheng <jian.cheng.1983@gmail.com>
  %