Add Unit Testing

[cdalzell]

There are a number of tests and code examples in the documentation, it would probably be helpful to create actual unit tests out of these to help expedite and increase the confidence in library data validation.

There are a number of packages out there, but this one (https://r-pkgs.had.co.nz/tests.html) seems to have CRAN integration.

[friendly]

I don't see any strong need to do this in a package that is 95% data