Original repository.
https://github.com/raulmur/ORB_SLAM2

I made some simple modifications relative to the original repository.

1.Clean up code, style changes.
2.Migrate DBow2 to DBow3.
3.Replace Dense and Eigen Solver to CSparse Solver.
4.Replace PnpSolver to cv::solvePnPRansac.
5.Migrate to opencv 4.0.
6.Migrate g2o to the latest version.
7.Added map reading and saving functions. I have tested it in monocular and stereo mode. referenced from https://www.cnblogs.com/mafuqiang/p/6972342.html.
8.Fix a bug (maybe is not bug) in ORBmatcher.cc . referenced from https://www.zhihu.com/question/35116055.
9.Fix a bug in Frame::Frame (Stereo). In the original implementation, the variable mb was used without initialization.
10.Add keyframes and map points to erase from memory. It's very difficult to add this feature, and maybe there are still bugs.

//TODO