For some inexplicable reason I failed to include the departure of flatness of the focal plane in the camera geometry. We are going to need this for some approaches to PSF modelling, and certainly for studies of the LSST camera during commissioning. In the short term, we need it to map out the HSC focal plane from focus sweep data (although that is no concern on the LSST project, it is relevant to studies of PSF estimation using HSC).

I propose that we call the height of the centre of the detector (defined as the point returned by Detector.getCenter()) Detector.piston and that it be measured in mm for consistency with Detector.pixelSize. If Detectors are flat this is sufficient as the Orientation is sufficient to define tilted devices, but we might want to future-proof cameraGeom by adding a representation of warped devices at the same time as this ticket.

Looking at the code to construct cameras it isn't obvious to me how to add a new field like this to cameraGeom (even with a default value of 0.0) in a non-intrusive way. If it is indeed impossible, the scope of this ticket must include suitable modifications of all obs packages (for some suitable value of "all": lsstSim, hsc, cfht, sdss, decam?)