Completed review of Chapter 8 Improving Our Model

Generally modified grammar and sentence arrangement in places.

Added @@dothis annotations.

For "Handling Beam Path" moved the test before the class redefinition to add the instance variable.

Corrected some code - why are we bothering to change API method names from the original e.g. #exitSideFor:  --> #exitFor: .  I understand changing some tests-method-names as a once-off change, but changing the API method names would seem to create work all throughout the book.

Provided some example code for the class-method alternative suggested in "Discussion About Directions".

Just an aside...I'm not sure I like how the terms "directions" and "sides" are used in the original site - but for now I've mostly left it as is and will see how it goes in later chapters.  It might be too big to change throughout the book. By way of example, I consider a "north direction" to be a beam "travelling north" that in a BlankCell will exit the "north side" whereas a beam entering a "south side" will exit a "north side" as shown in the code of this chapter.