fix: model classes (#167)

### Summary of Changes

Fix some issues with the initial implementation of the classes for
tables, columns, and rows:

* Import them in `__init__.py` file so they can be used elsewhere.
* Add leading underscore to attribute names to indicate that they are
only for internal use.
* Turn class attributes into instance attributes.
* Add a dummy test for CI (can be removed once actual tests are
available).

Runtime/safe-ds/tests/test_dummy.py

@@ -0,0 +1,2 @@
+def test_dummy():
+    pass