input: Write a python function to calculate the greatest common divisor (GCD) of two numbers using recursion.
  assert gcd(10, 25) == 5
  assert gcd(14, 21) == 7
  assert gcd(100, 200) == 100
output: def gcd(a, b):
      if b == 0:
          return a
      else:
          return gcd(b, a % b)