FREEtree                Initial FREEtree call which then calls actual
                        FREEtree methods depending on parameters being
                        passed through.
FREEtree_PC             Version of FREEtree called when fixed_regress
                        is NULL, uses principal components (PC) as
                        regressors for non-grey modules.
FREEtree_time           Version of FREEtree called when var_select and
                        fixed_regress are specified,
get_split_names         Method for extracting names of splitting
                        features used in a tree.
