Better code formatting with ``python

There is more than that here. I had a bad commit of a bunch of temporary
files, I had to reset back in time, and now I am doing a massive commit.
Sorry.

pdf/clean_book.bat

@@ -5,8 +5,12 @@ rm --f *.toc
 rm --f *.aux
 rm --f *.log
 rm --f *.out
-
 rm --f book.ipynb
 rm --f book.toc
 rm --f book.tex
+
+rm --f chapter.ipynb
+rm --f chapter.pdf
+
 rmdir  /S /Q book_files
+rmdir  /S /Q chapter_files