input: Write a function to design a data structure in Python that can store integer values. The data structure should support adding values and finding the minimum value stored in it. If no values have been added, the minimum value should return None.
  
  class IntList:
      def __init__(self):
          self.values = []
     
      def add_value(self, value):
          self.values.append(value)
      
      def find_min(self):
          if not self.values:
              return None
          return min(self.values)
  
  assert isinstance(IntList(), IntList)==True
  assert IntList().find_min()==None
  intList = IntList()
  intList.add_value(5)
  intList.add_value(3)
  intList.add_value(7)
  assert intList.find_min()==3
  intList.add_value(2)
  assert intList.find_min()==2
  assert IntList().add_value(10).add_value(8).find_min()==8
output: 
  class IntList:
      def __init__(self):
          self.values = []
     
      def add_value(self, value):
          self.values.append(value)
          return self
      
      def find_min(self):
          if not self.values:
              return None
          return min(self.values)
  