Sampler S coordinate

[Laurie Nevay]

We store “S” in the samplers in the event tree. We should get rid of this as it is a fixed number for a given sampler and does not at all need to be stored per sampler per event.

We need it for optics analysis, but it should be stored in the model tree once.

Similarly with z. I will remove this immediately as it has no knock-on effect for the optics analysis just now.