Add a `confint` method to use t-distribution based quantiles about mmrm HOT 2 CLOSED

when using df_1d, is it also appropriate to report the confidence intervals?

ideas is that we use df_1d and df_md for testing, for df_1d the d.f., estimate and distribution also provided, but lack the CI. it might be helpful to also report the CI

basically maybe we can create a function like this

confint.mmrm_tmb <- function(...) { #for confidence intervals of the coefficients
# call df_1d to obtain the d.f., est, etc and summarize them into a data.frame
}

for differences in marginal means, use

confint(pairs(emmeans(fit)))

from mmrm.

@clarkliming hm good question, let me think about it

from mmrm.