【398】COMP9021 - Polynomial

构建 Polynomial 类，实现 +, -, *, / and +=, -=, *=, /=

参考：如何用python编程求解二元一次方程组。如x+y=3;x-y=1

参考：python对重载运算符的限制

参考：python:自定义对象的打印

operator	overwrite
+	_add_
-	_sub_
*	_mul_
/	_truediv_
+=	_iadd_
-=	_isub_
*=	_imul_
/=	_itruediv_
字符串打印	_str_

改写举例：

def __add__(self, other):
    ...
    return Polynomial(self.ex + other.ex)

def __iadd__(self, other):
    return Polynomial(self.ex) + Polynomial(other.ex)