input: Write a function that adds two numbers and then doubles the result.
  assert double_add(5, 3)==16
  assert double_add(0, 0)==0
  assert double_add(-1, 1)==0
  assert double_add(2.5, 2.5)==10.0
  assert double_add(-1, -1)==-4
  def double_add(num1, num2): 
      return (num1 + num2) * 2
output: def double_add(num1, num2): 
      return (num1 + num2) * 2