(1) list of covariates FLAME performs matching at each iteration, (2)
Sizes, conditional average treatment effects (CATEs), and variance (if compute_var = TRUE)
of matches at each iteration, (3) match quality at each iteration, and (4) the original
data with additional column *matched*, indicating the number of covariates each unit is
matched on. If a unit is never matched, then *matched* will be 0.