Consume tokens.
        Args:
            tokens (float): number of transport tokens to consume
        Returns:
            wait_time (float): waiting time for the consumer

def consume(self, tokens):
        """Consume tokens.
        Args:
            tokens (float): number of transport tokens to consume
        Returns:
            wait_time (float): waiting time for the consumer
        """
        wait_time = 0.
        self.tokens -= tokens
        if self.tokens < 0:
            self._get_tokens()
        if self.tokens < 0:
            wait_time = -self.tokens / self.fill_rate
        return wait_time