input: Write a python function to find the maximum value in a list of integers.
  assert get_max_value([1, 2, 3, 4, 5]) == 5
  assert get_max_value([-10, -20, -5, -1]) == -1
  assert get_max_value([100, 200, 300, 400]) == 400
output: def get_max_value(lst):
      if not lst:
          return None
      max_value = lst[0]
      for num in lst:
          if num > max_value:
              max_value = num
      return max_value