input: Write a python function to calculate the cumulative sum of a list of numbers.
  assert cumulative_sum([1, 2, 3, 4]) == 10
  assert cumulative_sum([10, 20, 30, 40]) == 100
  assert cumulative_sum([5, 10, 15, 20]) == 50
output: def cumulative_sum(numbers):
      return sum(numbers)