Export `grid.draw()` methods allowing use of `ggsave()`

[ddsjoberg]

What changes are proposed in this pull request?

• Added S3 methods grid.draw.ggsurvfit() and grid.draw.ggcuminc() which in turn allows us to save images from the package directly with ggplot2::ggsave() (#107)

If there is an GitHub issue associated with this pull request, please provide link. closes #107

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

• [x] Ensure all package dependencies are installed by running renv::install()
• [x] PR branch has pulled the most recent updates from master branch. Ensure the pull request branch and your local version match and both have the latest updates from the master branch.
• [x] If a new function was added, function included in _pkgdown.yml
• [x] If a bug was fixed, a unit test was added for the bug check
• [x] Run pkgdown::build_site(). Check the R console for errors, and review the rendered website.
• [x] Overall code coverage remains >99.5%. Review coverage with withr::with_envvar(new = c("NOT_CRAN" = "true"), covr::report()). Begin in a fresh R session without any packages loaded.
• [x] R CMD Check runs without errors, warnings, and notes
• [x] 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 "# ggsurvfit (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).
• [ ] Increment the version number using usethis::use_version(which = "dev")
• [ ] Run codemetar::write_codemeta()
• [ ] Run usethis::use_spell_check() again
• [ ] Approve Pull Request
• [ ] Merge the PR. Please use "Squash and merge".

[bailliem]

This could be an issue with my work machine, but might need to update the renv snapshot to include ggeasy. My windows install is throwing an error when building the site as the ggeasy package is missing. It dosen't look like the package is part of the renv lock file . I don't see where its being used. Its resolved once I install the package.

-- RMarkdown error -------------------------------------------------------------
Quitting from lines 202-211 (gallery.Rmd) 
Error in loadNamespace(x) : there is no package called 'ggeasy'
In addition: Warning messages:
1: Tried to calculate with group_by(), but the calculation failed.
Falling back to ungrouped filter operation... 
2: Using `across()` in `filter()` is deprecated, use `if_any()` or `if_all()`. 
--------------------------------------------------------------------------------
Error in render_rmarkdown(pkg, input = input, output = output_file, output_format = format,  : 
  Failed to render RMarkdown
Caused by error:
! callr subprocess failed: there is no package called 'ggeasy'
Caused by error:
! there is no package called 'ggeasy'
Error: callr subprocess failed: Failed to render RMarkdown
Caused by error:
! callr subprocess failed: there is no package called 'ggeasy'
Caused by error:
! there is no package called 'ggeasy'
Type .Last.error.trace to see where the error occurred

[ddsjoberg]

Hey @bailliem ! Thank you for the review!

• I added a section to the gallery about saving the images.
• Regarding the ggeasy pkg: The gallery is an article, rather than a vignette. The distinction is that an article is not shipped with the package. In the gallery article we use ggeasy and gghighlight. Because it's not a vignette, these are not proper dependencies of the package (which is great!). I am not sure if there is a way to tell R that these should be installed at any point, though. We're not using renv here to track dependencies.