Given list of integers and integer n implement a function which calculates the maximum sum of n consecutive
elements in the list (sum of n digits next to another that have the largest sum).
You can use helper max function to deal with Kotlin nullability.
| Challenge | Solution |
maxSubListSum(listOf<Int>(), 3) // null
maxSubListSum(listOf(4, 2, 7), 2) // 9
maxSubListSum(listOf(4, 2, 7, 5, 8, 9, 5, 1), 3) // 22