Add definition of the order of functions (Iris, Prism, Blades...) from light source to lens

[danielmuellerma]

At the moment it can not be defined whether for example an iris is positioned in front of or behind the prism. There should be the possibility to define the order of functions that affect the beam from the light source to the light output

[petrvanekrobe]

Theoretically, geometry chain might be able to contain it, but there might be situations where the chain branches out and might not catch this. I think we should try to model this on few examples.