pdf_use_GramCharlier

代码说明：

  用著名的Gram-Charlier级数展开画变量的概率密度曲线或累积分布曲线。在程序中可选。需要用到6阶半不变量。(Use the famous painting Gram-Charlier series expansion to plot the probability density curve or cumulative distribution curve. Need to use 6-order half-invariant.)