Method iterator

uno/symbolic/VectorExpression.hpp:17–17

         using value_type = std::pair<size_t, double>;

         iterator(const VectorExpression& expression, size_t index): expression(expression), index(index) { }

         [[nodiscard]] std::pair<size_t, double> operator*() const {
            const auto [_, expression_index] = expression.indices.dereference_iterator(this->index);