a group metadata module is missing

[d-v-b]

array metadata is defined in core/metadata/v2.py and core/metadata/v3.py. By contrast, group metadata class is defined in core/group.py. This asymmetry is weird. We should put group metadata in core/metadata, even if we don't distinguish v2 and v3 group metadata.

I noticed this while working on #3012

[K-Meech]

Just adding a +1 for this - as I was about to open the same issue 😅 It would be great to have all of the metadata together in one place including GroupMetadata and ConsolidatedMetadata