Make loop closures more robust by potentially using scan context

[aserbremen]

Check out:

https://github.com/gisbi-kim/scancontext_tro

[aserbremen]

or any other global discriptor like m2dp?

matlab: https://github.com/LiHeUA/M2DP
c++: https://github.com/gloryhry/M2DP-CPP