input: Write a python function to sum all even numbers in a list.
  assert sum_even_numbers([1, 2, 3, 4, 5]) == 6
  assert sum_even_numbers([10, 21, 32, 43, 54]) == 96
  assert sum_even_numbers([7, 11, 23, 36, 44]) == 80
output: def sum_even_numbers(numbers):
      return sum(num for num in numbers if num % 2 == 0)