input: Write a python function to double each element in an array.
  assert double_array([1, 2, 3, 4, 5]) == [2, 4, 6, 8, 10]
  assert double_array([0, -1, -2, -3, -4]) == [0, -2, -4, -6, -8]
  assert double_array([10, 20, 30, 40, 50]) == [20, 40, 60, 80, 100]
output: def double_array(arr):
      return [x * 2 for x in arr]