check-in working versions of series.py and ack.py; need more work on the...

... asserts
Figuring out how to interact with the user is important; we'll use it a lot in upcoming assignments. However, for this assignment we really want to see unit testing using `assert` statements. The basic structure should be something like this:

    if __name__ == "__main__":
        # assert function(input) == expected
        # ...
        assert fibonacci(7) == 13
        # ...
        assert lucas(4) == 7
        # ...
        assert sum_series(4, 2, 1) == lucas(4)
        # ...
        print('all tests passed!')

The above code isn't ideal though as it can be improved to reduce repetition.