Use a serif font for latex documents

[abmantz]

Transfered over from LSSTDESC/start_paper#73

@ehneilsen writes:

While sans serif is great on overheads and documents to be read on low resolution screens, it is harder to read and can look unprofessional when used in a printed technical or scientific document. Can we replace the sans font in the latex style with a serif one?

[abmantz]

I think @drphilmarshall was going for a uniform look among the different possible substrate formats of Notes, and Markdown uses sans serif. (I'm also not sure anyone cares about that these days.)

It looks like the responsible lines are 84-85 in lsstdescnote.cls:

\usepackage{helvet}
\renewcommand{\familydefault}{\sfdefault}

You could play around with commenting these. Maybe we can eventually make this a class option.