Expand unit tests

[florian-huber]

There is now continuous integration running all test in /tests/ and the current tests already cover a big part of the code:

---------- coverage: platform linux, python 3.8.12-final-0 -----------
Name                          Stmts   Miss  Cover
-------------------------------------------------
memo_ms/__init__.py               7      0   100%
memo_ms/__version__.py            1      0   100%
memo_ms/classes.py              117     46    61%
memo_ms/import_data.py           47     17    64%
memo_ms/visualization.py        125    111    11%
tests/test_classes.py            62      0   100%
tests/test_user_workflow.py      35      0   100%
-------------------------------------------------
TOTAL                           394    174    56%
Coverage XML written to file coverage.xml

Next steps could be:

• Add tests for visualization
• Add more meaningfull test(s) for MemoMatrix class to test_classes.py
• Revisit user workflow test (test_user_workflow.py)