coinbase.go
func (c Client) GetExchangeRate(from string, to string) (float64, error) {
	exchanges, err := c.GetExchangeRates()
	if err != nil {
		return 0.0, err
	}
	key := from + "_to_" + to
	if exchanges[key] == "" {
		return 0.0, errors.New("The exchange rate does not exist for this currency pair")
	}
	exchangeFloat, err := strconv.ParseFloat(exchanges[key], 64)
	if err != nil {
		return 0.0, err
	}
	return exchangeFloat, nil
}
func (c Client) GetExchangeRate(from string, to string) (float64, error) {
	exchanges, err := c.GetExchangeRates()
	if err != nil {
		return 0.0, err
	}
	key := from + "_to_" + to
	if exchanges[key] == "" {
		return 0.0, errors.New("The exchange rate does not exist for this currency pair")
	}
	exchangeFloat, err := strconv.ParseFloat(exchanges[key], 64)
	if err != nil {
		return 0.0, err
	}
	return exchangeFloat, nil
}
// GetExchangeRate gets the exchange rate between two specified currencies