added unit test

tests/test_flip_image.py

+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()