Internal helper to detect empty columns (or rows) in symmetric input–output style tables.
non_zero_columns_find(data_table)
A logical value: TRUE
if the column contains at least one non-zero
numeric entry, or if the input is a factor/character column; FALSE
otherwise.
A column (vector) from a symmetric input–output table, a use table, or a supply table. May also be a factor or character vector.