For each column in a data.frame-like object, identify the hierarchical
level as the highest grouping level from groups_df for which the column is
identical for all members of every group.
get_datlvls(data, groups_df)A named character vector of length ncol(data). Each element is the
name of the grouping level at which the corresponding data column is
constant.
a data.frame
the data.frame of (integer) grouping vectors of length
nrow(data).
get_grouping_levels