Implement Schur-Complement for Anchored Landmarks

[strasdat]

ScaViSLAM uses at the moment an anchored landmark model. In general, each Observation is linked to three Jacobian:

• The point-Jacobian,
• The keyframe-Jacobian (from which the point is observed)
• The anchorframe-Jacobian (to which the point is anchored) Thus, we do have a "three" hyper-edge.

However, the current g2o release only supports the Schur-Complement for binary constraints. ToDo: Implement the Schur-Complement for 3-edges inside of g2o and integrate this change into ScaViSLAM. This should lead to a significant speed-up of the back-end!

(Note: At the moment, g2o is developed in a private repository. On openslam.org, there are only release snapshots. Modifying the version on openslam.org is probably not a good idea, because it is quite outdated already. However, there are plans to host the g2o development branch publicly soon...)