input: Write a python function to find the greatest common divisor (GCD) of two numbers.
  assert greatest_common_divisor(12, 24) == 12
  assert greatest_common_divisor(48, 18) == 6
  assert greatest_common_divisor(7, 5) == 1
output: def greatest_common_divisor(a, b):
      while b:
          a, b = b, a % b
      return a