Cleaning SmartStereoProjectionPoseFactor

[ToniRV]

• ToDo:
• Split .h/.cpp
• Use const&, avoid needless and costly copies
• Lint everything to 80 char otw it's annoying to read.
• Add asserts where the user could mess up badly (i.e. different sizes for measurements and keys, etc)
• had to add gtsam_unstable to examples... Can I avoid that? Yes, moved the example to gtsam_unstable
• calibration() returns a weird const vector, if const of the K_ elements was the intended purpose, the const should be inside the shared_ptr... otherwise the original const is useless.
• Add a unique Id (Key) to the smart factor, otw there is a ton of bookkeeping to be done by the user (perhaps in another PR)

[ProfFan]

@ToniRV There is a dedicated examples for unstable:
https://github.com/borglab/gtsam/tree/develop/gtsam_unstable/examples

[ToniRV]

@ProfFan I guess the problem then is that examples/ISAM2_SmartFactorStereo_IMU.cpp
should be in the gtsam_unstable/examples folder instead of examples?
Since it uses:

#include <gtsam_unstable/slam/SmartStereoProjectionPoseFactor.h>

[dellaert]

I think that’s right. Feel free to move it!
thanks for this PR!

[dellaert]

I’d move the example to unstable, but other than that, looks great!

[dellaert]

Cool. I'll merge.