Add repeats argument (#327)

[mikemahoney218]

I accidentally pushed this to main 🤦 I reverted immediately (git reset --soft HEAD~1 && git push -f) and believe things are fine, but sorry about that.

That said! This PR fixes #327 by:

• Adding a repeats argument to group_vfold_cv()
• Adding an error to both group_vfold_cv() and vfold_cv() when repeats is used with v == max_v

library(rsample)

vfold_cv(mtcars, v = 2, repeats = 2)
#> #  2-fold cross-validation repeated 2 times 
#> # A tibble: 4 × 3
#>   splits          id      id2  
#>   <list>          <chr>   <chr>
#> 1 <split [16/16]> Repeat1 Fold1
#> 2 <split [16/16]> Repeat1 Fold2
#> 3 <split [16/16]> Repeat2 Fold1
#> 4 <split [16/16]> Repeat2 Fold2
vfold_cv(mtcars, v = nrow(mtcars), repeats = 2)
#> Error in `vfold_cv()`:
#> ! Repeated resampling when `v` is 32 would create identical resamples

group_vfold_cv(mtcars, cyl, v = 2, repeats = 2)
#> # Group 2-fold cross-validation 
#> # A tibble: 4 × 3
#>   splits          id      id2      
#>   <list>          <chr>   <chr>    
#> 1 <split [11/21]> Repeat1 Resample1
#> 2 <split [21/11]> Repeat1 Resample2
#> 3 <split [14/18]> Repeat2 Resample1
#> 4 <split [18/14]> Repeat2 Resample2
group_vfold_cv(mtcars, cyl, v = length(unique(mtcars$cyl)), repeats = 2)
#> Error in `group_vfold_cv()`:
#> ! Repeated resampling when `v` is 3 would create identical resamples
group_vfold_cv(mtcars, cyl, v = NULL, repeats = 2)
#> Error in `group_vfold_cv()`:
#> ! Repeated resampling when `v` is `NULL` would create identical resamples

^{Created on 2022-07-01 by the reprex package (v2.0.1)}

[juliasilge]

Thank you! 🙌

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