Write is_nondesc() to check whether variables are in non-descending order

[kelly-sovacool]

Similarly to close_enough(), I like to use is_nondesc() paired with the assertthat package to make sure that claims I make in a manuscript are true. If you re-run your code or a pipeline, numbers could change slightly, and you'll want to be sure that your overall conclusions did not change. For example, if I make a claim that "x is less than y and y is less than z", then I also include a hidden code chunk with assert_that(is_nondesc(x, y, z)).

Checklist

(~Strikethrough~ any points that are not applicable.)

• [x] Write unit tests for any new functionality.
• [x] Update roxygen comments & vignettes if there are any API changes.
• [x] Update NEWS.md if this includes any user-facing changes.
• [x] The check workflow succeeds on your most recent commit.

[sklucas]

This sounds like a sound method for checking your work.