PD plot not showing on multiclass

[ledell]

If you use wine dataset with "quality" and let it train a classification (multiclass model), the PD plot is broken. That's because we are missing the part where we set a reference class, so we need to update the code to input a reference class.

For multiclass, we should add a second picker box with the list of classes, so that you can choose what class you view the PD plot for.