Initial package infrastructure

[pratikunterwegs]

This PR makes initial changes to the template.

To do:

• Check the package title (must be title case, but please check content), see DESCRIPTION;
• Check the package description field, and check that the reference is suitable, and suggest others if needed, see DESCRIPTION and also README.Rmd (the plain markdown will render automatically via an action);
• Check and suggest changes to the author list.
• Any other issues: license file, pkg maintainer (I am currently listed), pkg author contact details (I have deliberately omitted these for now).

Everything can be changed later, so no need to consider any text final.

This branch will auto-delete once this PR is merged. I think a develop/main branching workflow is sufficient for initial package development. We can discuss branch naming and issue creation once there's a minimal working example.

If merging directly on approval, please prefer a rebase to keep a linear commit history.

[zegibney]

Thanks for this Pratik! I've added some initial thoughts/ comments but would wait and see what Xiang/Katy have to say too before making any substantial changes :)

[xiangli313]

Thanks @pratikunterwegs and @zegibney. The PR looks great to me. I would agree with Zoe's comments in general.

[pratikunterwegs]

There's a MacOS workflow failing for non-package reasons (it can't install something upstream) - so this should be safe to review. I'll wait a bit before re-running the failing workflow, and merge once it passes.