add return statements to python binding wrappers

some overridden functions were not returning when they should be

src/python/bindings_docstring_keyed.hpp

@@ -54,8 +54,8 @@ namespace sisso
                 inline std::string get_postfix_term(){return this->get_override("get_postfix_term")();}
                 inline void update_add_sub_leaves(std::map<std::string, int>& add_sub_leaves, int pl_mn, int& expected_abs_tot){this->get_override("update_add_sub_leaves");}
                 inline void update_div_mult_leaves(std::map<std::string, double>& div_mult_leaves, double fact, double& expected_abs_tot){this->get_override("update_div_mult_leaves");}
-                inline int n_feats(){this->get_override("n_feats");}
-                inline std::shared_ptr<Node> feat(int ind){this->get_override("feat");}
+                inline int n_feats(){return this->get_override("n_feats");}
+                inline std::shared_ptr<Node> feat(int ind){return this->get_override("feat");}
             };
             /**
              * @brief struct used wrap an OperatorNode object for conversion