input: Write a function to find the maximum number in a list.
  assert find_maximum([11, 9, 12, 8, 5]) == 12
  assert find_maximum([3, 5, 7, 2, 8]) == 8
  assert find_maximum([-1, -3, -4, -2, -5]) == -1
output: def find_maximum(numbers):
      if not numbers:
          return None  # Assuming returning None for an empty list
      max_num = numbers[0]
      for num in numbers[1:]:
          if num > max_num:
              max_num = num
      return max_num