fix decision tree equality and move default constructor to public

gtsam/discrete/DecisionTree-inl.h

@@ -79,7 +79,7 @@ namespace gtsam {
     bool equals(const Node& q, double tol) const override {
       const Leaf* other = dynamic_cast<const Leaf*> (&q);
       if (!other) return false;
-      return std::abs(double(this->constant_ - other->constant_)) < tol;
+      return this->constant_ == other->constant_;
     }
 
     /** print */

gtsam/discrete/DecisionTree.h

@@ -113,14 +113,14 @@ namespace gtsam {
     convert(const typename DecisionTree<M, X>::NodePtr& f, const std::map<M,
         L>& map, std::function<Y(const X&)> op);
 
-    /** Default constructor */
-    DecisionTree();
-
   public:
 
     /// @name Standard Constructors
     /// @{
 
+    /** Default constructor (for serialization) */
+    DecisionTree();
+
     /** Create a constant */
     DecisionTree(const Y& y);