Hierarchical grouping of IRFs

[kosack]

The problem we have is that there are many "types" of IRFs that may be associated with one EventList:

• spatial selection (point-like vs general IRFs)
• event types
• ... others?

While these could be "dimensions" of the IRF, that quickly becomes difficult from a memory footprint standpoint, so to allow "lazy loading" of the IRF tree, we need some way to group and name them.

We could for example use the FITS Hierarchical Grouping Standard, or invent our own