input: Write a python function to find the maximum value in a list of numbers.
  assert find_maximum([1, 2, 3, 4, 5])==5
  assert find_maximum([10, 20, 5, 30, 15])==30
  assert find_maximum([7, 9, 23, 0, 4])==23
output: def find_maximum(numbers):
      if not numbers:
          return None
      max_val = numbers[0]
      for num in numbers[1:]:
          if num > max_val:
              max_val = num
      return max_val