input: Write a python function to find the greatest product of any two numbers in an array.
  assert calculate_greatest_product([-10, -10, 5, 2]) == (-10, -10)
  assert calculate_greatest_product([1, 10, 2, 6, 5, 3]) == (10, 6)
  assert calculate_greatest_product([1, 3, 4, 7, 9]) == (7, 9)
output: def calculate_greatest_product(arr):
      arr.sort()
      product1 = arr[-1] * arr[-2]
      product2 = arr[0] * arr[1]
      if product1 > product2:
          return arr[-2], arr[-1]
      else:
          return arr[0], arr[1]