ctree_leaf_distributions() should use the classnames not classes

[parrt]

Line 1549 of trees.py func ctree_leaf_distributions() should use the classnames not classes:

ax.legend([bar_container0, bar_container1],
          [f'class {self.shadow_tree.classes()[0]}', f'class {self.shadow_tree.classes()[1]}'])

[parrt]

Actually, it looks like that function is also limited to binary Target where as it should be any number of target. And I guess the code can simply call add_classifier_legend() instead of creating its own legend.

[mepland]

@parrt fixed in #221