General wishes for improved beam slam

[jakemclaughlin6]

1. Better “lost” detection and restarting of the front end
2. Visual place recognition in global mapping
3. Map merging in global mapping
   1. When the front end gets lost and restarts, the global mapper should try to merge the maps
4. Better covariances between vo and lo
5. Publish better graph visualization
   1. marker array with special color edges between poses

Just writing this here to keep track