Add support for 2d cameras and robot-held checkerboards

[mikeferguson]

Requires:

• CheckerboardFinder that puts 2d points in data rather than 3d
• chain3d_to_camera2d_error block - whereas all the other error blocks use the project() function of both models to move all the points into 3d and then compare them, this error block would project() the chain into 3d points, and the project those 3d points into the 2d camera using the optical model. The residual would be the difference between the projected 2d camera points and the observed points from the 2d camera. Requires adding inverse projection function to models.