vpda: add getter for discrimination tree

Author: mtf90

algorithms/active/discrimination-tree-vpda/src/main/java/de/learnlib/algorithms/discriminationtree/vpda/AbstractVPDALearner.java

@@ -93,6 +93,10 @@ public OneSEVPA<?, I> getHypothesisModel() {
         return hypothesis;
     }
 
+    public DTree<I> getDiscriminationTree() {
+        return dtree;
+    }
+
     protected static <I> void link(DTNode<I> leaf, HypLoc<I> loc) {
         assert leaf.isLeaf();
         leaf.setData(loc);