input: Write a python function to find the maximum number in a list.
  assert find_maximum([7, 3, 12, 9, 4]) == 12
  assert find_maximum([1, 2, 3, 4, 5]) == 5
  assert find_maximum([-1, -2, -3, -4, -5]) == -1
output: def find_maximum(lst):
      if not lst:
          return None
      max_num = lst[0]
      for num in lst[1:]:
          if num > max_num:
              max_num = num
      return max_num