input: Write a python function to sum all numbers in a list.
  assert sum_nums([1, 2, 3, 4, 5]) == 15
  assert sum_nums([10, -2, 3, 5, 9]) == 25
  assert sum_nums([0, 0, 0, 0]) == 0
output: def sum_nums(nums):
      return sum(nums)