Tagged release for 4.3.0

[ProfFan]

Hi Daniel, we are about to make a tagged release, do you have anything (PR) that you want that is unmerged?

[DanMcGann]

WRT iMESA the only GTSAM changes that I use that are not currently in 4.2.0 are from this PR (borglab/gtsam#1504) which simply aligns the Pose2 and Pose3 accessor interface to make writing code templated on Poses easier.

IMO it would be a good thing to include regardless of its use here since those changes do not change the interface in a way that can break existing projects that use GTSAM, but the changes do expand functionality in a useful way.