LightGBM tree parameters

Also written: num_leaves, min_child_samples, subsample, subsample_freq, colsample_bytree, reg_lambda, reg_alpha

The main LightGBM controls: `num_leaves` (leaves per tree, i.e. complexity), `min_child_samples` (fewest rows in a leaf), `subsample` with `subsample_freq` (random share of rows per tree; does nothing unless `subsample_freq` is at least 1), `colsample_bytree` (random share of columns per tree) and `reg_lambda`/`reg_alpha` (L2/L1 penalties on leaf values).