Add version check on package load

[andrewmarx]

When the package is loaded via the library() function, check the installed version and compare to the newest version available on CRAN. Output a startup message to the user informing them if a new version is available and how to update the package.

[andrewmarx]

Potentially useful info for when I get around to this:

• Relevant functions: https://stat.ethz.ch/R-manual/R-devel/library/base/html/ns-hooks.html
  • It appears that implementing .onAttach() would be the place to do this
  • .onLoad() should not be used for this, and should not have output messages
  • onAttach() should not load libraries. This may complicate things.
  • packageStartupMessage() should be used to output the info to the user
• Relevant SO question: https://stackoverflow.com/questions/33837615/how-to-access-a-librarys-function-in-onattach
• Example: https://rdrr.io/cran/camtrapR/src/R/onAttach.R
• Blog post: https://web.archive.org/web/20160414203215/http://thecoatlessprofessor.com/programming/automatically-check-if-r-package-is-the-latest-version-on-package-load/
  • A dead link was given the example in the previous point. This is an archived version of it.