WS-2751: Fix WS utilities unit tests

This work involved:

1. Writing some unit tests for database methods (database.py) to learn about writing unit tests.
2. Bug fixes and formatting changes in the metadata wrester (aat_wrest.py).
3. Formatting changes in metadata wrester tests (test_metadata_wrester.py).
4. Rewriting unit tests for core_sampler.py, including putting them in a test class.
5. Creating a test class for test_row_writer.py, putting existing test methods into this test class, and formatting the file so it passes Pylint and 'black' code checks.
6. Modifications of how testing is done in test_contacts_wrester.py and test_observation_wrester.py.
7. A slight modification to contacts_wrester to allow the passing in of a cursor (used in unit tests).