input: Write a function to calculate the cumulative sum of a list of numbers.
  assert cum_sum([1, 2, 3, 4, 5]) == [1, 3, 6, 10, 15]
  assert cum_sum([10, -2, 4, 7, 0]) == [10, 8, 12, 19, 19]
  assert cum_sum([5, 10, 15]) == [5, 15, 30]
output: def cum_sum(numbers):
      cum_sum_list = []
      total = 0
      for number in numbers:
          total += number
          cum_sum_list.append(total)
      return cum_sum_list