input: Write a python function to find the maximum value in a list of numbers.
  assert maxValue([5, 9, 8, 2, 1, 3]) == 9
  assert maxValue([10, 20, 30, 40, 50]) == 50
  assert maxValue([-1, -2, -3, -4, -5]) == -1
output: def maxValue(numbers):
      if not numbers:
          return None
      max_val = numbers[0]
      for num in numbers:
          if num > max_val:
              max_val = num
      return max_val