Misleading subgeometry error message in egs_chamber

[ftessier]

Describe the issue

When not using any subgeometry in egs_chamber, the message initScoring: missing/wrong 'sub geometries' for geometry... is echoed, leading the user to think that this is a mistake, whereas subgeometries are optional. Documentation on subgeometry is unclear.

To Reproduce

Run any egs_chamber simulation without subgeometries.

Proposed solution

• [ ] Clarify message when there are no subgeometry, instead of missing. If the code can distinguish between none, missing, and wrong, then echo separate messages and branch accordingly.

• [ ] Improve documentation.