input: Write a function to calculate GCD and LCM of two numbers.
  assert gcd_lcm(8, 12) == (4, 24.0)
  assert gcd_lcm(15, 20) == (5, 60.0)
  assert gcd_lcm(7, 13) == (1, 91.0)
output: def gcd_lcm(a, b):
      import math
      gcd = math.gcd(a, b)
      lcm = (a * b) / gcd
      return gcd, lcm