added unit test for demo function

csst_proto/some_other_modules.py

-def an_arbitrary_functions(*args):
-    """ an arbitrary function """
-    print("Hi, this is an arbitrary function.")
-    for _ in args:
-        print(_)
+def a_demo_function(*args):
+    """ a demo function """
+    return None

csst_proto/top_level_interface.py

 from .flip_image import flip_image, read_test_image
-from .some_other_modules import an_arbitrary_functions
+from .some_other_modules import a_demo_function
 
 
-__all__ = ["flip_image", "read_test_image", "an_arbitrary_functions"]
+__all__ = ["flip_image", "read_test_image", "a_demo_function"]

tests/test_other_functions.py

+import unittest
+
+from csst_proto.top_level_interface import a_demo_function
+
+
+class TestCaseDemoFunction(unittest.TestCase):
+    def test_demo_function(self):
+        # flip test image
+        self.assertTrue(
+            a_demo_function(1) is None
+        )