input: Write a function that takes two numbers as input, adds them together, and then doubles the result. The function should return the final result. The function header should be `def double_add(num1, num2)`. Include the following test cases with the expected output:
  
  assert double_add(3, 4) == 14
  assert double_add(0, 0) == 0
  assert double_add(-1, 1) == 0
  assert double_add(-5, -3) == -16
  assert double_add(10, 20) == 60
output: def double_add(num1, num2):
      return (num1 + num2) * 2