Add relevant constants for USD and GBP

src/currencyExchange.h

@@ -1,7 +1,9 @@
 #include <stdio.h>
 
-#define CURRENT_USD_RATE_OF_ONE_EURO 1.07
-#define CURRENT_GBP_RATE_OF_ONE_EURO 0.89
+#define USD_RATE_OF_ONE_EURO 1.07
+#define GBP_RATE_OF_ONE_EURO 0.89
 
-float toUSD(float euro);
-float toGBP(float euro);
+#define CURRENCY_CODE_USD 1
+#define CURRENCY_CODE_GBP 2
+
+float convert(float euro, int newCurrencyCode);