input: Write a function to check if a number is prime.
  assert PrimeChecker().is_prime(5) == True
  assert PrimeChecker().is_prime(4) == False
  assert PrimeChecker().is_prime(13) == True
output: class PrimeChecker:
      def is_prime(self, num):
          if num <= 1:
              return False
          if num <= 3:
              return True
          if num % 2 == 0 or num % 3 == 0:
              return False
          i = 5
          while i * i <= num:
              if num % i == 0 or num % (i + 2) == 0:
                  return False
              i += 6
          return True