input: Write a function to design a data structure in Python to store integer values. The data structure should be able to add integer values to it and find the minimum value among the stored integers. If there are no values stored, the find_min method should return None.
  class IntList:
      def __init__(self):
          self.values = []
     
      def add_value(self, value):
          self.values.append(value)
      
      def find_min(self):
          return min(self.values) if self.values else None
  
  assert IntList().find_min()==None
  assert IntList().add_value(5).find_min()==5
output: 
  class IntList:
      def __init__(self):
          self.values = []
     
      def add_value(self, value):
          self.values.append(value)
          return self
      
      def find_min(self):
          return min(self.values) if self.values else None
  