Added Amount.isNegative and Collection<Amount>.sum() extension methods

BankingClientModel/src/commonMain/kotlin/net/codinux/banking/client/model/Amount.kt

@@ -12,6 +12,19 @@ fun Amount.toFloat() =
 fun Amount.toDouble() =
     this.toString().toDouble()
 
+val Amount.isNegative: Boolean
+    get() = this.toString().startsWith("-")
+
+fun Collection<Amount>.sum(): Amount {
+    var sum: Amount = Amount.Zero
+
+    for (element in this) {
+        sum += element
+    }
+
+    return sum
+}
+
 
 @NoArgConstructor
 expect class Amount(amount: String = "0") {