Use 'testthat' to keep our package in shape

[timtreis]

As briefly discussed in Slack, we could use testthat to perform various checks to keep our codebase in line with CRAN requirements. Additionally, if we decide to advertise our package's riskmetric score, we could also integrate checks for that.

CRAN:

• No T/F, but TRUE/FALSE
• ...

riskmetric:

• "NEWS file contains entry for current version number"
• has_vignettes
• has_bug_reports_url
• export_help
• has_website
• r_cmd_check [Broken?]
• remote_checks [Broken?]
• has_maintainer
• has_news
• has_source_control
• covr_coverage [Broken?]

[timtreis]

I propose to have a small number of dedicated testthat files acting as watchdogs for potential issues or metrics we want to monitor or ensure. If we want to stick to a 0/0/0 policy for each PR, this will then automatically ensure we also stick to the other standards and metrics we agreed to.

Such a watchdog for the feedback we got from the CRAN team has already been merged in PR #237.

A similar one could somewhat easily be written for compliance with the metrics that riskmetric is evaluating.

Potential negative aspect: Bloats the codebase a bit.