Method iterator

uno/linear_algebra/SparseVector.hpp:22–22

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

         iterator(const SparseVector& vector, size_t index): vector(vector), index(index) { }

         value_type operator*() const {
            return {this->vector.indices[this->index], this->vector.values[this->index]};