input: Write a python function to find the maximum number in a list.
  assert find_maximum([25, 12, 15, 32, 7]) == 32
  assert find_maximum([5, 3, 9, 1, 4]) == 9
  assert find_maximum([-1, -5, -3, -4]) == -1
  assert find_maximum([100, 200, 300, 400, 500]) == 500
output: def find_maximum(lst):
      if not lst:
          return None
      max_num = lst[0]
      for num in lst:
          if num > max_num:
              max_num = num
      return max_num