修改的drosta2（给出降水量计算干旱时长、烈度、开始年月），原上传者原晨阳，原文件中无p_m方程，无法使用，加入并加入导出数据方程.(Revised drosta2(a package could give drought-length, strngth, and start month/yeat based on giiven monthly mean precipitation). This is revised based on the package uploaded by Yuan