构造大数运算类 +；-；*；

1. 不考虑括号

2. 输入数据最大长度为40 位,要捕获异常

3. 不考虑输入小数部分-Construction of Large Numbers computing class+ - *　1. Without regard to bracket 2. Input data the maximum length of 40-bit, to catch the exception 3. Without regard to decimal part