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, -2, 3, 4]) == 15
  assert sum_array([7, 7, 7, 7, 7, 7]) == 42
output: def sum_array(arr):
      return sum(arr)