Add a vignette / article on working with CHI numbers

[Moohan]

Inspired by the question on Teams this morning. I wrote an article / vignette to which goes hopefully explains, with examples, all the functions which do things with CHI numbers.

The nicest way to view this is as an article with pkgdown, where you can see it as it will look on the online documentation.

pkgdown::init_site()
pkgdown::build_article("chi-operations")
pkgdown::preview_page('articles/chi-operations.html')

Alternatively/additionally to see it as a vignette in R:

devtools::build_vignettes()
devtools::load_all()
vignette("chi-operations", package = "phsmethods")

@Tina815 This can wait for a future version if you want s I know you're very close to finalising 0.2.3!

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (106aaa6) 97.40% compared to head (0c4eb4f) 97.40%. Report is 3 commits behind head on dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #106 +/- ## ======================================= Coverage 97.40% 97.40% ======================================= Files 13 13 Lines 424 424 ======================================= Hits 413 413 Misses 11 11 ```

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