WIP template for PNAS

[alcrene]

As I started writing a draft for PNAS, I thought I could start by porting their template so I can write the article in MyST.

It is… not the most fun LaTeX class to work with, but I mostly got it to work. Current state is here: https://github.com/alcrene/myst-template-pnas2023
There are still unresolved points, e.g.

• compilation uses the wrong engine (XeTeX instead of pdflatex)
• lack of support for template-specific commands or conventions:
  • \SCFigure for figures with a caption to the side.
  • Wrapping equations with \begin{figure*} to make them span both columns.

A full list of todos/limitations is in the Readme.

Following your documentation, I’m opening an issue to propose contributing the template into the official collection.

(And by the same token, possibly get some help making the template more robust in time for when I will be ready to submit 😉 )