Add generalised function for currency exchange

src/currencyExchange.c

@@ -13,3 +13,13 @@ float toGBP(float euro) {
 
     return GBP;
 }
+
+float convert(float euro, int newCurrencyCode) {
+    switch(newCurrencyCode) {
+        case 1:
+            return ( euro * CURRENT_USD_RATE_OF_ONE_EURO );
+        case 2:
+            return ( euro * CURRENT_GBP_RATE_OF_ONE_EURO );
+    }
+    return -1;
+}