A vector of names of importance measures to be calculated - if equal to NULL then all are calculated;
if "p_value" is to be calculated then "no_of_nodes" will be too. Suitable measures for classification forests are:
mean_min_depth, accuracy_decrease, gini_decrease, no_of_nodes,
times_a_root. For regression forests choose from: mean_min_depth,
mse_increase, node_purity_increase, no_of_nodes, times_a_root.