* temporary fix for eliminate_implications

* Change from << to <==

* Fixed implies operator in SAT_plan

* changed Expr name to counter

* Fixed a typo in logic.py

* Added translate_to_SAT()

* extract solution from model

* added test cases

* removed debug code

Add InfixOps, refactor expo and Expr

* changes a typo of file name to /aima-data

* modified some documentaion and removed doc tests

* changed the names like Fig[2, 3] and added unit tests for search.py

* adds Fig[4.23] graph which is 1-dim state space problem

* adds LRTA star agent class

* adds fully implemented LRTA star agent

* adds tests for LRTA star agent