input: Write a python function to sum all elements in an array.
  assert sum_array([1, 2, 3, 4, 5]) == 15
  assert sum_array([10, 20, 30, 40, 50]) == 150
  assert sum_array([5, -5, 10, -10, 20]) == 20
output: def sum_array(arr):
      return sum(arr)