Create higher-level functions #5

The vignette currently contains a lot of code that would be better abstracted away from users as higher-level functions.
https://github.com/bschilder/ThreeWayTest/blob/a4c61c94ed339778355ffe0f406849bf54f1da22/vignettes/ThreeWayTest.Rmd

This will ultimately make the software easier to use (and harder to misuse) with less verbosity and redundant code. You can still export the subfunctions they run to users, but making these into discrete functions.