input: Write a python function to sum the elements of an array.
  assert sum_array([1, 2, 3, 4, 5]) == 15
  assert sum_array([-1, -2, -3, -4, -5]) == -15
  assert sum_array([0, 0, 0, 0, 0]) == 0
output: def sum_array(arr):
      return sum(arr)