Create Folds for Cross-Fitting

`make_folds(y, V = 2, stratified = FALSE, C = NULL, probs = rep(1/V, V))`

- y
the outcome

- V
the number of folds

- stratified
should the folds be stratified based on the outcome?

- C
a vector indicating whether or not the observation is fully observed; 1 denotes yes, 0 denotes no

- probs
vector of proportions for each fold number

a vector of folds