input: Write a python function to find the maximum value in a list of numbers.
  assert get_max([3, 4, 5, 7, 10]) == 10
  assert get_max([-1, -2, -3, -4]) == -1
  assert get_max([1, 2, 3, 4, 5]) == 5
output: def get_max(numbers):
      if not numbers:
          return None
      max_value = numbers[0]
      for number in numbers:
          if number > max_value:
              max_value = number
      return max_value