How do we support 2d camera to 2d camera calibration?

[mikeferguson]

This is an extension of https://github.com/mikeferguson/robot_calibration/issues/41, but is quite a bit more involved. This will however, require encoding the 3d-points that are being back projected into both cameras somewhere in the state estimation of the free parameters. Mixing this block with camera2d_to_chain3d could also produce some wonky stuff, if the chain3d projection does not match the magic state estimation....