A mable (model table) data class (mdl_df) is a tibble-like data structure
for applying multiple models to a dataset. Each row of the mable refers to a
different time series from the data (identified by the key columns). A mable
must contain at least one column of time series models (mdl_ts), where the
list column itself (lst_mdl) describes how these models are related.
mable(..., key = NULL, model = NULL)