Hierarchical module/block definition

[KrisThielemans]

#22 introduced Modules of detector elements (single level). However, modules are generally hierarchical (blocks, mini-blocks, crystals etc, or even "units of modules"). This would be good to encode e.g. for dead-time.

[MaxTousss]

I can't find the exact PR, but this was used in the last NssMic, so this issue could be closed, right?

[KrisThielemans]

The issue is a question if we need/want to implement more levels than we currently have, so cannot close it quite yet.

However, in recent discussions, we have come to a tentative conclusion that we don't need to. A "PETSIRD module" would then be the smallest level that is needed (for dead-time, singles histograms etc). Higher levels can then be constructed "by hand" by just listing them via the rigid transformations. It's not clear to me yet if this breaks some stuff in the SGID concept for symmetries (but I don't think so), or if it implies a storage overhead.

So, I'll leave this up for a bit longer.