input: Write a function to find the maximum value in a list.
  assert find_maximum([3, 5, 7]) == 7
  assert find_maximum([10, 20, 30, 40, 50]) == 50
  assert find_maximum([1, -1, 0, -10, 100]) == 100
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