[FEAT] Implement PspA Plots into MolEvolvR RPackage

[the-mayer]

Description

This PR will introduce 'PspA' plots into the MolEvolvR package, addressing #90

What kind of change(s) are included?

• Feature (adds or updates new capabilities)
• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (these changes would cause existing functionality to not work as expected).

Checklist

Please ensure that all boxes are checked before indicating that this pull request is ready for review.

• I have read and followed the CONTRIBUTING.md guidelines.
• I have searched for existing content to ensure this is not a duplicate.
• I have performed a self-review of these additions (including spelling, grammar, and related).
• I have added comments to my code to help provide understanding.
• I have added a test which covers the code changes found within this PR.
• I have deleted all non-relevant text in this pull request template.
• Reviewer assignment: Tag a relevant team member to review and approve the changes.

[the-mayer]

@eboyer221, I've checked in the plotting code from the molevol_scripts repository.

Please integrate this into plotting.R with appropriate

• documentation (for both functions)
• imports
• roxygen2 generated .Rd files
• tests, where applicable

Code should pass devtools::check(). This code may need a few additional helper wrappers, which do not need to be exported to the user namespace.

Let me know if you have questions -- thanks!