GitLab

* Docstring for udf

* Track udf docs

* Improve

* Improved

* Delete udf.py

* Improve

* add examples in traversal.py

* message propagate methods

* use the new message propagation for tree-lstm

* update to the new name

* update propagate API doc

* update doc

* add propagate utest

* refactor API doc; add markup link in tutorials

* fix readme