Ensure that the tutorial works with the ax backend

[crasanders]

The fully worked visual psychophysics experiment demonstrates all the main features of AEPsych, but it has only been tested with the old backend. We should make sure it works with the Ax backend by doing the following:

1. Add use_ax = True to the config string.
2. Run the notebook and verify that the experiment still works.
3. Verify that the database can be loaded and replayed.
4. Verify that we can plot the data using AEPsychStrategy.plot_slice. We won't get the threshold since Ax currently doesn't support plotting thresholds, but the shape of the function should still look the same.

If any of these steps does not work, we need to identify where things break down in the code and how to fix it.