S3 class for the output of twfeCovs(). Carries the same
styled print / summary / coef surface as the three
sibling estimators, plus tidy / glance (broom) and
simultaneousCIs. plot and augment are intentionally
not provided: twfeCovs() estimates one pooled effect per cohort, so
it has no per-(cohort, time) / event-study basis to plot, and its
coefficient vector is in a reduced basis that augment()'s
fitted-value path does not match (#58). Both raise an informative error.