Fix implementation of multiple unique fold columns for repeated cross-validation about groupdata2

[LudvigOlsen]

Fixed both of these.

Then found that you can use unique(as.matrix(data), MARGIN=2) to do a similar thing and test against current approach:

code:

`set.seed(1)
df <- data.frame("participant" = factor(rep(c('1','2', '3', '4', '5', '6'), 3)),
"age" = rep(c(25,65,34), 3),
"diagnosis" = rep(c('a', 'b', 'a', 'a', 'b', 'b'), 3),
"score" = c(34,23,54,23,56,76,43,56,76,42,54,1,5,76,34,76,23,65))

df <- df %>% dplyr::arrange(participant, score)

system.time({
df_folded_100reps <- fold(df, 3, num_col = 'score', num_fold_cols=100,max_iters = 100)
})
`
Current approach:
user system elapsed
16.939 0.266 17.310

Using unique:
user system elapsed
247.794 4.186 253.402

So sticking to my own approach.
One reason for the difference may be, that I only compare two columns once, while unique can compare two columns up to 100 times in the example.