add stop_if_ utility functions to test func args

[cjyetman]

• depends on make it a package #59
• depends on add R CMD check action and a badge to the README #60
• closes add unit testing with testthat #52
• closes add utility functions for checking function arguments and config parameters #53

also adds the infrastructure for unit testing

[jacobvjk]

lgtm

[cjyetman]

@jacobvjk you "reviewed" and commented, but didn't "approve"