实验：
有两个个矩阵a和b，均为2行3列。 重载运算符“+”，
（1）使之能用于矩阵的相加。如c=a+b。
（2）如果参与运算的两个操作数中一个是整数即矩阵+3或3+矩阵形式，进行加法运算也能得出和。
（3）定义自减运算符的前置（i++）和后置（++i）重载编写相应程序并验证它的正确性。
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