iLBA or similar structureless bundle adjust

[vadixidav]

See this link for details: https://borg.cc.gatech.edu/projects/ilba.html.

Structureless bundle adjust is a technique that is already used in vslam-sandbox today. Unfortunately, the way that it does it is not efficient. It runs Nelder-Mead optimization using all the landmark observations as constraints for all the views. By splitting the reconstruction into smaller constraints, such as with iLBA, we might be able to scale better than we do today.