input: Write a function to generate a Python program that adds two numbers and then doubles the result. The function should take two input parameters, add them, double the sum, and return the final result.
  assert double_add(5, 3)==16
  assert double_add(10, 20)==60
  assert double_add(-3, -7)==-20
  assert double_add(0, 0)==0
output: def double_add(a, b):
      return (a + b) * 2