a data.table containing the values to look up
as well as values associated with the looked-up values
that need to be returned.
col_name_for_lookup_values
in the reference data.table,
name of the column containing lookup_values.
col_name_for_output_values
in the reference data.table,
name of the column containing values to return (i.e., values
associated with the looked-up values that will be the function's output)