C#GridView控件导出Excel及多层表头类，说明：此类是集合了网上朋友的一些类方法拼写而成，发表出来一便朋友们借鉴

关于GridView的多种使用方式，如GridView加入自动求和求平均值小计。GridView数据导入Excel/Excel数据读入GridView-GridView use on a variety of ways, such as the GridView automatically adding the average for Sub-total sum. GridView data into Excel/Excel data read into the GridView, etc.