Alternative 'About Design Points'

Coming back to this work after a few days off, I was confused for a fe minutes why ==MirrorCell>>initializeExitSides== only instantiated the Dictionary and did not fill it.  In re-familiarising myself with it, I came to the opinion that ==leanLeft== should be called from ==MirrorCell>>initializeExitSides== rather than ==MirrorCell>>initialize==.  

This would make ==initialize== identical between the three cell classes (and incidentally all removed when only a call to ==super initialize== remains.) I updated the text to suit.

There was some design 'alternative' presented in the previous chapter, that would complicate things here, so I removed that.

btw, I thought I'd have a go at working in a feature branch of the SBA repository.