def concat other_df
      vectors = (@vectors.to_a + other_df.vectors.to_a).uniq

      data = vectors.map do |v|
        get_vector_anyways(v).dup.concat(other_df.get_vector_anyways(v))
      end

      Daru::DataFrame.new(data, order: vectors)
    end
Concatenate another DataFrame along corresponding columns.
If columns do not exist in both dataframes, they are filled with nils