以字典形式返回分级B数据
        :param fields:利率范围，形如['+3.0%', '6.0%']
        :param min_volume:最小交易量，单位万元
        :param min_discount:最小折价率, 单位%
        :param forever: 是否选择永续品种,默认 False

def fundb(self, fields=None, min_volume=0, min_discount=0, forever=False):
        """以字典形式返回分级B数据
        :param fields:利率范围，形如['+3.0%', '6.0%']
        :param min_volume:最小交易量，单位万元
        :param min_discount:最小折价率, 单位%
        :param forever: 是否选择永续品种,默认 False
        """
        if fields is None:
            fields = []
        # 添加当前的ctime
        self.__fundb_url = self.__fundb_url.format(ctime=int(time.time()))
        # 请求数据
        rep = requests.get(self.__fundb_url)
        # 获取返回的json字符串
        fundbjson = json.loads(rep.text)
        # 格式化返回的json字符串
        data = self.formatfundbjson(fundbjson)
        # 过滤小于指定交易量的数据
        if min_volume:
            data = {
                k: data[k]
                for k in data
                if float(data[k]["fundb_volume"]) > min_volume
            }
        if len(fields):
            data = {
                k: data[k]
                for k in data
                if data[k]["coupon_descr_s"] in "".join(fields)
            }
        if forever:
            data = {
                k: data[k]
                for k in data
                if data[k]["fundb_left_year"].find("永续") != -1
            }
        if min_discount:
            data = {
                k: data[k]
                for k in data
                if float(data[k]["fundb_discount_rt"][:-1]) > min_discount
            }
        self.__fundb = data
        return self.__fundb