[WIP] Refactor planning.py (#921)

* GraphPlan fixed

* Updated test_planning.py

* Added test for spare_tire

* Added test for graphplan

* Added shopping problem

* Added tests for shopping_problem

* Updated README.md

* Refactored planning notebook

* Completed shopping problem

* Refactors

* Updated notebook

* Updated test_planning.py

* Removed doctest temporarily

* Added planning graph image

* Added section on GraphPlan

* Updated README.md