input: Write a python function to compute the sum of elements in an array.
  assert sum_array([2, 4, 6, 8]) == 20
  assert sum_array([1, 2, 3, 4, 5]) == 15
  assert sum_array([10, -2, 3, -1]) == 10
output: def sum_array(arr):
      return sum(arr)