Introduce boxed means classification

This PR introduces boxed means classification. It computes intervals according to the successive means of the generated subsets. As described in #449, the number of intervals changes according to the chosen method.

There was a small bug I fix here too. When creating a classification, the "Create" button was disabled while the classification that had been used was still selected (and not "Manual", consequently). This was due to a problem in the initialization order in PnlAbstractCategorized#getCreateClassificationPanel.