def uniq
      uniq_vector = @data.uniq
      new_index   = uniq_vector.map { |element| index_of(element) }

      Daru::Vector.new uniq_vector, name: @name, index: new_index, dtype: @dtype
    end
def uniq
      uniq_vector = @data.uniq
      new_index   = uniq_vector.map { |element| index_of(element) }

      Daru::Vector.new uniq_vector, name: @name, index: new_index, dtype: @dtype
    end
Keep only unique elements of the vector alongwith their indexes.