Create pdf in build_pdf directory?

[rjleveque]

@ketch: I suggest we

• consolidate make_chapters.py and make_book.sh into make_pdf.py similar to make_html.py so it's less confusing where things are done and where the specify the desired chapters.

• Build the pdf file in a new directory build_pdf so that all the files generated 0*.ipynb, riemann.aux, etc. don't end up cluttering up the main directory. Then move the pdf file out of this directory so it can be safely deleted to clean up. This would be similar to what's done in #83 for html files.

[ketch]

Yes, my original implementation was hacky and this sounds good. I will try to do this tomorrow.