def add *indexes
      Daru::CategoricalIndex.new(to_a + indexes)
    end
Add specified index values to the index object
@param indexes [Array<object>] index values to add
@return [Daru::CategoricalIndex] index object with added values
@example
idx = Daru::CategoricalIndex.new [:a, :b, :a, :b, :c]
idx.add :d
# => #<Daru::CategoricalIndex(6): {a, b, a, b, c, d}>