Method copy

src/TopicModel/TopicModel.hpp:623–628

		TopicModel(const TopicModel&) = default;

		std::unique_ptr<ITopicModel> copy() const override
		{
			auto ret = std::make_unique<_Derived>(*static_cast<const _Derived*>(this));
			ret->updateForCopy();
			return ret;
		}

		size_t getNumDocs() const override
		{