input: Write a function to create a class in Python named Calculator with two methods: add_two_numbers and add. The add_two_numbers method should take two parameters, a and b, and return the sum of a and b. The add method should take three parameters, a, b, and c, and return the sum of a, b, and c.
  assert calculator.add_two_numbers(1, 2) == 3
  assert calculator.add_two_numbers(-1, 1) == 0
  assert calculator.add(1, 2, 3) == 6
  assert calculator.add(-1, -2, -3) == -6
  assert calculator.add_two_numbers(0, 0) == 0
output: class Calculator:
      def add_two_numbers(self, a, b):
          return a + b
      
      def add(self, a, b, c):
          return a + b + c