sub.by: criteria for subclassification. Choose from:
"treat" (default), the number of treatment units;
"control", the number of control units; or "all",
the total number of units.
subclass: either a scalar specifying the number of
subclasses, or a vector of probabilities bounded between 0 and 1,
which create quantiles of the distance measure using the units in
the group specified by sub.by (default = subclass =
6).