input: Write a python function to square each element in an array.
  assert square_array([1, 2, 3, 4, 5]) == [1, 4, 9, 16, 25]
  assert square_array([10, 20, 30]) == [100, 400, 900]
  assert square_array([0, -1, -2]) == [0, 1, 4]
output: def square_array(arr):
      return [x**2 for x in arr]