input: Write a function to find the greatest common divisor (GCD) of a list of numbers.
  assert find_GCD([48, 64, 80]) == 16
  assert find_GCD([100, 200, 300]) == 100
  assert find_GCD([7, 13, 19]) == 1
output: def find_GCD(numbers):
      from math import gcd
      from functools import reduce
      return reduce(gcd, numbers)