added unit test

import unittest

import numpy as np

from csst_proto.top_level_interface import flip_image, read_test_image

class FlipImageTestCase(unittest.TestCase):

    def test_flip_image(self):

        # flip test image

        self.assertTrue(

            np.all(flip_image(read_test_image()) == np.array([[4, 3], [2, 1]]))

        )

        # the code fails for 1D array

        with self.assertRaises(AssertionError):

            flip_image(np.array([1, 2, 3, 4]))

if __name__ == "__main__":

    unittest.main()