Write function to ensure / create proper group coding

[sanoke]

[Related to #24]

Proper subgroup coding means that this variable takes consecutive integer values, starting at 1. Groups are numbered in order of increasing subgroup-specific average treatment effect (ATE).

At the moment, proper coding is an assumption, and I need to write a function to create/enforce it. If there's an empty subgroup / invalid subgroup number, no plots will display. (There's a notice to this effect in the panel.)

Write function to check that grouping variable is coded properly. Do group reassignment if needed.
In the interim, I'll provide a script for the user to do this check themselves. Eventually I will do this check within my application, and do some reassignment as needed.