input: Write a python function to find the greatest common divisor (GCD) of two numbers using recursion.
  assert gcd(30, 15) == 15
  assert gcd(100, 25) == 25
  assert gcd(27, 14) == 1
  assert gcd(56, 98) == 14
output: def gcd(a, b):
      if b == 0:
          return a
      else:
          return gcd(b, a % b)