Add a leave-group-out resampling method

[topepo]

Similar to caret's groupKFold.

[topepo]

There are a bunch of tests that now pass (tibbles!).

Please test and/or evaluate to give feedback.

Here example is:

> set.seed(3527)
> test_data <- data.frame(id = sort(sample(1:20, size = 80, replace = TRUE)))
> test_data$dat <- runif(nrow(test_data))
> 
> set.seed(5144)
> split_by_id <- group_vfold_cv(test_data, group = "id")
> 
> get_id_left_out <- function(x)
+   unique(assessment(x)$id)
> 
> library(purrr)
> table(map_int(split_by_id$splits, get_id_left_out))

 1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 
 1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1 
> 
> set.seed(5144)
> split_by_some_id <- group_vfold_cv(test_data, group = "id", v = 7)
> held_out <- map(split_by_some_id$splits, get_id_left_out)
> table(unlist(held_out))

 1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 
 1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1 
> # number held out per resample:
> map_int(held_out, length)
1 2 3 4 5 6 7 
3 3 3 3 3 3 2 

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