Added PartialOrderPlanner (#927)

* Added PartialOrderPlanner

* Added doctests

* Fix doctests

* Added tests for PartialOrderPlanner methods

* Added test for PartialOrderPlanner

* Rerun planning.ipynb

* Added notebook section for TotalOrderPlanner

* Added image

* Added notebook section for PartialOrderPlanner

* Updated README.md

* Refactor double tennis problem

* Refactored test for double_tennis_problem

* Updated README.md

* Added notebook sections for job_shop_problem and double_tennis_problem

* Updated README.md

* Fixed refinements example

* Added go_to_sfo problem

* Rename TotalOrderPlanner

* Renamed PDDL to PlanningProblem