Pre GTSAM 4.2

This tag pins the latest commits on DC-SAM main before the migration to GTSAM 4.2a8, the current latest GTSAM version (thanks to @plusk01 for migrating main)

If your project depends on a version of GTSAM before its 4.2a8 release, then you should use this tag. You'll want to use specifically GTSAM at commit caa14bc, which is the version we used in our initial DC-SAM testing. Other GTSAM commits may work, too, but caa14bc is the one that has been tested :)