Following a suggestion by C.G.Vedant, changed |implies| to |’==>’|

* removes obsolete Fig dictionary from utils.py

* modified rl notebook according to the new Figure conventions

* removes 'import Fig' and changed the names of Figures

* fixes a small error (by me) in executing the cells in rl notebook

from utils import implies

* Made Log Usage Consistent in text.py

* Added Explicit Imports from utils

* 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

- Fix relative imports in base of is a package or not
- Fix not working notebooks
- Add notebooks to packages sources

Used tool 2to3 to finish the porting to Python 3 plus some fixes by
hand.

These functions, which I had to implement in Python 2.2, are now
available in standard Python.

use “raise NotImplementedError” for abstract methods