【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)