def add_category(*new_categories)
      new_categories -= categories
      add_extra_categories new_categories
    end
Associates a category to the vector.
@param [Array] new_categories new categories to be associated
@example
dv = Daru::Vector.new [:a, 1, :a, 1, :c], type: :category
dv.add_category :b
dv.categories
# => [:a, :b, :c, 1]