input: Write a python function to calculate the Simple Moving Average (SMA) for a given list of values and window size.
  assert SMA(3).calculate([10, 20, 30, 40, 50]) == 140 / 3
  assert SMA(4).calculate([5, 10, 15, 20, 25, 30]) == 110 / 4
  assert SMA(2).calculate([1, 2, 3, 4, 5]) == 9 / 2
output: def SMA(window_size):
      class SMA_Calculator:
          def __init__(self, window_size):
              self.window_size = window_size
              
          def calculate(self, values):
              if len(values) < self.window_size:
                  return None
              return sum(values[-self.window_size:]) / self.window_size
  
      return SMA_Calculator(window_size)