TODO: this should not be user-facing
tmb_params_trans(model, vec_type = c("tmb_fun_arg", "params", "tv_mult"))flexmodel
type of vector to return:
(1) tmb_fun_arg = same length as the argument for TMB objective function,
(2) params = same length as the params element in model
(3) tv_mult = same length as the model$timevar$piece_wise$schedule$init_tv_mult