New FocusGenerator

[SAnsell]

I have merged some work by Tsito who has built a Dream/Skadi/trex model. In those models, he has used a 4 layer guide structure and the existing FocusGenerator effectively hard coded 3 only.

I have fixed this mess and regressed the changes back through all the instruments -- so you can have as many layers as you wish BUT the interface has changed.

If this causes problems in local branches, you will see that you need a setLayer(N,thickness,material) ,rather than the old setGuideMaterail() and setThickness(A,B,C).