For V-fold type cross-validation. This takes a fold object and returns a fold
vector (containing the validation set IDs) for use with other tools like
cv.glmnet.
folds2foldvec(folds)A fold object as produced by make_folds,
from which a numeric vector of the validation set fold IDs are
returned.
Other fold generation functions:
fold_from_foldvec(),
fold_funs,
make_folds(),
make_repeated_folds()