Reject loop closures by pose distances

[Skwangles]

A particular use case of mine is tracking movements through an office building which has fairly repetitive hallways. It is fantastic using g2o and works great. A weakness in my use case is that occasionally a loopclosure can occur between floors vertically and take everything out of alignment.

Initally I though the 'reject_by_graph_distance' was this feature, however, after digging found it was just based on the essential graph. Whether it be a feature request (which I am happy to contribute code towards, if pointed to the right function in the loop_detector.cc module), or just a personal modification, I would like to implement a way to reject loop closures based on the estimated pose distance.

Value:
Adding an additional way to protect against prevent destructive loop closures in areas which have clear physical separation but are similar.

Details about possible implementation:
I was thinking about either splitting it that per usecase one could specify min/max x, y, z distance (or more simply the min/max pythag distance)
Current location I was trying to understand/thinking may be a good location was the 'loop_detector::select_loop_candidate_via_Sim3' or the 'loop_detector::detect_loop_candidates_impl'.