[MAINT] Add a default value to the threshold_selection_tool chart

[ThomasHepworth]

Is your proposal related to a problem?

The threshold_selection_tool chart is currently rendered without a default value, resulting in the chart not displaying the confusion matrix until the user hovers over the HTML output.

Describe the solution you'd like

Add a default threshold that the user can adjust. In the absence of this, set the default to point that maximises the f1 score.