Adds a sort stage to aggregation query
        :param kwargs: Specifies the field(s) to sort by and the respective sort order.
        :return: The current object

def sort(self, **kwargs):
        """
        Adds a sort stage to aggregation query
        :param kwargs: Specifies the field(s) to sort by and the respective sort order.
        :return: The current object
        """
        query = {}
        for field in kwargs:
            query[field] = kwargs[field]
        self._q.append({'$sort': query})
        return self