Release 4.3a2

[Dellaert, Frank]

Hi all, 

I released a new alpha version,  4.3a2, release notes here: https://github.com/borglab/gtsam/releases/tag/4.3a2

One of the key additions which should be useful for everyone in this group is the documentation and examples: https://borglab.github.io/gtsam/

Note: The API is not stable for these new additions. Esp. the GPU acceleration and certifiable optimization are still in flux. Once those settle down, we aim to cut a 4.3 release.

Many thanks for all your contributions, questions, and issues you filed. We could not do this without you!

Best!

Frank