public void ConvertListOfBalancesToGivenCurrency()
        {
            var balance1 = new CustomerBalance
            {
                Currency = (int)Currency.GBP,
                Balance  = 1000
            };
            var balance2 = new CustomerBalance
            {
                Currency = (int)Currency.USD,
                Balance  = 1000
            };

            var balances = new List <CustomerBalance> {
                balance1, balance2
            };
            var curr = new CurrencyConverter();
            var res  = curr.ConvertListOfBalancesToGivenCurrency(balances, Currency.PLN);

            output.WriteLine($"I got {res} PLN from those monies");
        }