Coherent handling of metadata for the IRF classes

[bkhelifi]

Currently the IRF classes have a meta dictionary. One should introduce a pydantic handling of the metadata,

[AtreyeeS]

What meta information should the IRF classes have? The DL4 classes created by Gammapy can have the Creation info. They can also store info of the type of DL3 products they were created from (eg: the HDUCLAS options), but isn't it more of provenance info?

[bkhelifi]

One has: the fixity, the context, maybe the simulation configuration or te provenance. Depending if the IRFs are stored into a single file, or into several files, the metadata can be spread.
When correctly read, these metada can be handled (ie reduced) for the DL4 products.