Revision 613

(Classification.AppendGroup,
        Classification.InsertGroup, Classification.ReplaceGroup,
        Classification.RemoveGroup, Classification.GetGroup): Do as the
        names imply.
(Classification.FindGroup): This was called GetGroup, but GetGroup
        takes an index, while FindGroup takes a value.
(Classification.__deepcopy__): Copy all the groups, BUT NOT THE LAYER
        REFERENCE. Currently there is a cyclic reference between the layer
        and its classification. If the classification doesn't need to know
        its owning layer we can change this, since it may make sense to be
        able to use the same classification with different layers.