Add Normalization Tool to main GUI toolbar

[AnneCarpenter]

Currently, all tools are listed in the menu item Tools and in the main GUI. However, Normalization Tool is missing from the GUI, which makes its existence easy to overlook. We should add it.

[DavidStirling]

Haha, I assumed there was a reason we had no GUI icon for this. It's still available in the toolbar menus.

In terms of normalization, the addition of optional scaling to the classifiers should make this redundant, but I can still imagine that someone might want to normalise their input table for other purposes.