input: Write a python function to calculate the greatest common divisor (GCD) of two numbers using recursion.
  assert gcd(48, 18) == 6
  assert gcd(101, 103) == 1
  assert gcd(56, 98) == 14
output: def gcd(a, b):
      if b == 0:
          return a
      else:
          return gcd(b, a % b)