HLRF_method

代码说明：

  一次可靠度的HLRF算法。随机向量的概率模型采用Nataf分布，考虑变量相关性。验算点搜寻采用U空间的梯度迭代算法。(This is a first-order reliability method based on HLRF algorithm. A multivariate distribution model,Nataf model,is used to generate the random variables. To search the design point in indenpent standard Gauss space, a kind of gradient iterative algorithm is adopted.)