def dup_only_valid vecs=nil
      rows_with_nil = @data.map { |vec| vec.indexes(*Daru::MISSING_VALUES) }
                           .inject(&:concat)
                           .uniq

      row_indexes = @index.to_a
      (vecs.nil? ? self : dup(vecs)).row[*(row_indexes - rows_with_nil)]
    end
Creates a new duplicate dataframe containing only rows
without a single missing value.