Run code conditionally to pass noSuggests check

[nanxstats]

Closes #243

This PR fixes the noSuggests check errors, by running code examples, tests, and vignettes conditionally, following the extrachecks recipe.

Now there are only two check notes when running the noSuggests check:

Packages suggested but not available for checking:
  'covr', 'dplyr', 'ggplot2', 'gsDesign', 'gsDesign2', 'gt',
  'survMisc', 'survRM2', 'tidyr'

* checking Rd cross-references ... NOTE
Package unavailable to check Rd xrefs: ‘gsDesign’

which I think are acceptable given they are direct benign side effects of not having Suggests packages installed when doing the check.

To run the noSuggests check locally, use R CMD check without any Suggests packages installed (except for knitr, rmarkdown, testthat) and set environment variable _R_CHECK_FORCE_SUGGESTS_=FALSE. I also used the GitHub Actions workflow mentioned in the extrachecks recipe and got consistent results.

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Also updated NEWS.md and bumped version number to 0.4.1.

[LittleBeannie]

Thanks!