Move remaining `pnas_article()` LaTeX into `template.tex`

[coatless]

I think the following should be moved inside the template.tex file:

<!-- Leave these lines as they are at the end of your .Rmd file to ensure placement of methods & acknowledgements sections before the references-->
\showmatmethods
\showacknow
\pnasbreak 

[eddelbuettel]

Yes. Good point. And I think I tested that they behave well when empty, ie not used.