Improve KB5 foils to have rectangular cross section

[Mateasek]

KB5 cameras are TargettedePixels with slit targets. The slits are rectangular, with very high ratio. This decreases the efficiency of sampling since a TargettedPixel launches rays into the bounding sphere projection.

I proposte to separate the slit into n segments with ratio of sides close to rectangle which should result in an improved sampling efficiency. Since TargettedPixel can have multiple targets, this shouldn't be an issue.

[jacklovell]

Good idea. Worth checking how much of a performance improvement you get with it: if the efficiency is indeed significantly better (I guess defined as mean/std for a set number of rays) then this will be worth doing for the bolometer models for most machines. But if the improvement is only small I'd value the simplicity and intuitiveness of the current setup.