input: Write a python function to find the maximum value in a list.
  assert find_maximum([5, 8, 0, 1, 3, 9]) == 9
  assert find_maximum([-1, -5, -3, -4, -2]) == -1
  assert find_maximum([100, 200, 300, 400, 500]) == 500
output: def find_maximum(lst):
      if not lst:
          return None
      max_value = lst[0]
      for num in lst:
          if num > max_value:
              max_value = num
      return max_value