input: Write a python function to create a Calculator class with add, subtract, and multiply methods.
  assert Calculator().add(2, 3) == 5
  assert Calculator().subtract(10, 4) == 6
  assert Calculator().multiply(3, 4) == 12
output: class Calculator:
      def add(self, a, b):
          return a + b
  
      def subtract(self, a, b):
          return a - b
  
      def multiply(self, a, b):
          return a * b