add_CVfolds_ind: Define a column of fold indicators for V-fold...
In osofr/GriDiSL: Discrete Super Learner with Grid-Search for Longitudinal Data

The input data is assumed to have repeated observations per subjects, the folds are defined as clustered by subject IDs.

1	add_CVfolds_ind(data, ID, nfolds = 5, fold_column = "fold", seed = NULL)

`data`	Input dataset, can be a `data.frame` or a `data.table`.
`ID`	A character string name of the column that contains the unique subject identifiers.
`nfolds`	Number of unique folds (same fold is always assigned to all observations that share the same ID).
`fold_column`	A name of the column that will contain the fold indicators
`seed`	Random number seed for selecting a random fold.