input: Write a function to create a Python class that modifies the case of a given string. The class should have methods to convert the string to uppercase, lowercase, and title case. If the input string is "Hello World", calling these methods should output "HELLO WORLD", "hello world", and "Hello World" respectively.
  assert StringCaseModifier('Hello World').to_upper()=='HELLO WORLD'
  assert StringCaseModifier('Hello World').to_lower()=='hello world'
  assert StringCaseModifier('Hello World').to_title()=='Hello World'
output: class StringCaseModifier:
      def __init__(self, input_string):
          self.input_string = input_string
      
      def to_upper(self):
          return self.input_string.upper()
      
      def to_lower(self):
          return self.input_string.lower()
      
      def to_title(self):
          return self.input_string.title()