input: Write a function to double the elements of an array.
  assert double_array([1, 2, 3, 4]) == [2, 4, 6, 8]
  assert double_array([0, -1, -2, 5]) == [0, -2, -4, 10]
  assert double_array([10, 20, 30]) == [20, 40, 60]
output: def double_array(arr):
      return [x * 2 for x in arr]