Add a first draft of stratification with groups

[mikemahoney218]

Hi all!

Following some conversation in #317 I figured I'd post a first draft of what stratification with grouping might look like. This first draft only handles group_vfold_cv() when balance = "groups".

This version requires strata to be constant across all groups. If users have a non-constant variable they want to use for stratification, I think it makes sense to force them to create the strata (as I do in the example), because any automated binning implement will probably be non-optimal.

This implementation differs from make_strata() in that strata doesn't need to be quoted.

I'm not sure how you feel about putting the arguments after ... -- because I only added strata and pool, and didn't include breaks, nunique, or depth, I didn't want to let people pass stratification arguments by position in case those other arguments get added later.

Addresses #317

[hfrick]

Looks great, thank you! 🎈 Some lines have gotten really long, could you break those up, please? I think we're good to go after that.

[mikemahoney218]

Sounds good! I merged your pillar branch into this one to make CI work.

[github-actions]

This pull request has been automatically locked. If you believe you have found a related problem, please file a new issue (with a reprex: https://reprex.tidyverse.org) and link to this issue.