aggregate
a character specifying how to aggregate
predictions or coefficients of single base learners. The default
returns the prediction or coefficient for the final number of
boosting iterations. "cumsum" returns a matrix with the
predictions for all iterations simultaneously (in
columns). "none" returns a list with matrices where the
jth columns of the respective matrix contains the
predictions of the base learner of the jth boosting
iteration (and zero if the base learner is not selected in this
iteration).