Check if a grouping variable varies within another grouping variable
check_groups_vary_within_lvl(lvl, grouping_df)a named logical vector indicating for each grouping variable
(column of grouping_df) whether it varies within the grouping
defined by lvl
a vector defining a grouping level
a data.frame with where each column contains the
grouping indices of a hierarchical level (as obtained
by get_groups(...))