学习和掌握运算符重载，理解异常处理机制。

实验内容：

　　（1）编程实现一个实数矩阵类，它可以进行加、减、乘和赋值运算，并且要重载运算符“（）”，用来返回矩阵元素的值。

（2）该类还要支持异常处理机制，能够处理可能的非法矩阵操作。-Learn and master operator overloading, understanding exception handling mechanism. Experiment: (a) programming a real matrix class, which can add, subtract, multiply, and assignment operator, and to override operator " ()"　is used to return the value of the matrix element. (2) This class also supports exception handling mechanism to deal with possible illegal matrix operations.