Add IBAN validation

WGneu/Utils.cs

@@ -21,5 +21,11 @@ namespace WGneu {
                     yield return childOfChild;
             }
         }
+
+        public static int Modulo(string a, int b) {
+            if (!a.All(char.IsDigit))
+                throw new ArgumentException("First argument has to be a decimal string");
+            return a.Select(ch => ch - '0').Aggregate((sum, n) => (sum * 10 + n) % b);
+        }
     }
 }