input: Write a python function to sum all the elements in an array.
  assert sum_array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) == 55
  assert sum_array([10, 20, 30, 40, 50]) == 150
  assert sum_array([5, 15, 25, 35, 45]) == 125
output: def sum_array(arr):
      return sum(arr)