Clarify error message when factor is required

**Summary** The error below is due to the fact that the `TRTA` column must be a factor and not a character vector. I think that it would be preferable if either: 1. The error message were clarified to tell which column in the input is leading to the error (i.e. tell that `TRTA` needs to be a factor rather than `coordinates[[1L]]`) 2. If acceptable, character inputs were automatically converted to factors. (Though, I understand that this may not be preferred since factor ordering is often important.) ** Reprex ** ``` r library(mmrm) library(tidyverse) d <- data.frame( USUBJID = factor(rep(1:6, each = 2)), TRTA = rep(c("A", "B"), 6) ) |> mutate( lnAVAL = log(case_when( TRTA == "A" ~ 100, TRTA == "B" ~ 120 )*exp(rnorm(n = n(), mean = 0, sd = 0.1))) ) mmrm( formula = lnAVAL ~ TRTA + us(TRTA|USUBJID), data = d ) #> Error in h_mmrm_tmb_data(formula_parts, data, weights, reml, singular = if (control$accept_singular) "drop" else "error", : Assertion on 'coordinates[[1L]]' failed: Must be of type 'factor', not 'character'. ``` ^{Created on 2025-11-20 with [reprex v2.1.1](https://reprex.tidyverse.org)}