References chapter subheader fix for psychology and across texts

Fixes #438 

For this pull request, pursuant to our discussion this morning w/Phil, Ed, Derek, I investigated the issue further.  Although the current issue we talked about still exists, and going forward I will place these types of things in the book.less file I think I have a solution for this one that's elegant.

Looking at all our books, I see that this subheading under references (Chapter 1, Chapter 2) is always in Serif font. For the chapter-area namespace in precalc, there isn't a font-family property, it simply inherits the body font which is Sans-Serif

My solution is here to add the font-family to the precalculus-slot,  then I also found in history-skeleton a reference to (#part>#book-end>#end-of-book-references>.style();) which can be removed.  By placing the font declaration in precalc-slots, in effect we allow the other books to inherit this.  At this point if we need to change this we can alter in history/econ slot.  

This would be an override which is ok (additive) and much better than a reset (subtractive)  For each fix, I will look at this way of doing things.... all roads leading to precalculus as they say.  

Also the (#chapter-area) namespace was carried over into psychology-slots to allow the font size to be set for these at 10pt per the psychology design template.  Again, if PreCalculus ends up needing this same size (precalc design template omits these so we don't know) we can move this slot property to precalc also and let all books inherit.