Calculate trend slopes per each covariate category

This is a feature request. With rtrim, one can apply a categorical covariate, and with `index(,covars = TRUE)` it can calculate indices for each covariate category. I would like to also calculate the trends for each covariate category. `coef()` doesn't do it, you would somehow have to sum each category with baseline (non-trivial with the standard errors). `overall()` doesn't seem to do it either.