Select variables based on the heuristic multiple selection algorithm from Kormaksson et al. 'Sequential
knockoffs for continuous and categorical predictors: With application to a large psoriatic arthritis clinical
trial pool.' Statistics in Medicine. 2021;1–16.
Simple knockoff generator based on least squares fit (continuous variables) or multinomial logistic regression (factor variables) respectively.
If X is empty, knockoffs are sampled from the marginal distribution of y
Knockoff (feature) statistics that captues the predictive strength: Absolute coefficient differences between treatment original variables interaction terms and treatment knockoff variables interaction terms