Point mutation documenation/function

[hfuchs5]

What changes are proposed in this pull request?
A new exported helper function which allows users to identify and rename specific point mutations.

If there is an GitHub issue associated with this pull request, please provide link.
#322

---

Reviewer Checklist (if item does not apply, mark is as complete)

• PR branch has pulled the most recent updates from main branch. Ensure the pull request branch and your local version match and both have the latest updates from the main branch.
• If a new function was added, function included in _pkgdown.yml
• If a bug was fixed, a unit test was added for the bug check
• Run pkgdown::build_site(). Check the R console for errors, and review the rendered website.
• Code coverage is suitable for any new functions/features. Review coverage with withr::with_envvar(new = c("NOT_CRAN" = "true"), covr::report()). Begin in a fresh R session without any packages loaded.
• R CMD Check runs without errors, warnings, and notes
• usethis::use_spell_check() runs with no spelling errors in documentation

When the branch is ready to be merged into master:

• Update NEWS.md with the changes from this pull request under the heading "# cbioportalR (development version)". If there is an issue associated with the pull request, reference it in parentheses at the end update (see NEWS.md for examples).
• Run codemetar::write_codemeta()
• Run usethis::use_spell_check() again
• Approve Pull Request
• Merge the PR