Triangulation support for SmartProjectionPoseFactor

[dellaert]

Feature

Someone (probably @chrisbeall-nuro back in the day) added smart triangulation capabilities to the SmartProjectionFactor. Is there a reason why this cannot also be integrated in the SmartProjectionPoseFactor and hence could be pushed to the base class?

Motivation

This would enable using smart factors with triangulation support when the camera calibration is fixed, and optionally a sensor offset is known.

Pitch

Push triangulation code into base class SmartFactorBase.

Additional context

There is an issue that in theory not all smart factors are about projections. Hence, it might need interposing an intermediate smart factor base class that is specialized to image projections and can benefit from triangulation.

[dellaert]

I found out by solving issue #101 that SmartProjectionFactor is already a base class of SmartProjectionPoseFactor, so it seems the functionality is already available. It's not clean, though, as it is strange to have a factor on two variables derive from a factor on one (other) variable. A common base class would be cleaner.