New functions when unit testing moved elsewhere

[pbbgss]

The existing functions were designed with the unit testing section in mind and a more complicated than they need to be for just demonstrating how to assemble a package.

I propose we replace them with simplified versions of is_friday() and glue_year_months(). These function should be easy to understand (thus document), have only two dependencies {stringr} and {lubridate} and would remove the need to cover the data/ s3 interactions in section 1.

In section 16 the "update" would be to add another function is_monday() which is almost identical to is_friday() so people should be able to adapt the tests.