https://qingsimon.github.io/post/2019-01-11-vins-mono%E5%9B%9B%E5%88%9D%E5%A7%8B%E5%8C%96/
https://github.com/ManiiXu/VINS-Mono-Learning/blob/master/vins_estimator/src/estimator.cpp
https://github.com/QingSimon/VINS-Mono-code-annotation/blob/master/vins_estimator/src/estimator.cpp