ReadDirections

% read direction file into a matrix
%
% USAGE:
%   grad = ReadDirections(filename)
%
% INPUT
%   filename :   direction file name (.txt)
%
%
% OUTPUT
%   grad     :  Nx3 matrix, each row is a point in sphere.
%
% Copyright (c) 2014, Jian Cheng <jian.cheng.1983@gmail.com>
%