HERD inside NWBFile/schema

[mavaylon1]

Motivation

Allow HERD to be stored internally, but still allow external override

• Test roundtrip internal HERD
• Test link_resources
• Test external HERD link through the IO herd path
• Test Add functionality where you provide herd path and it calls link_resources

How to test the behavior?

Show how to reproduce the new behavior (can be a bug fix or a new feature)

Checklist

• Did you update CHANGELOG.md with your changes?
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[mavaylon1]

Future development:
On the surface to the user, HERD is a table and when you query before any I/O, you will see lists and tuples. Under the hood, HERD is written as a structured array and on read is no longer a list/tuple but a structured array. This was a bit surprising and so I spent some time to map over the array to a list on read in the get_item. However, this grew in complexity reaching __swap_refs in AbstractH5TableDataset. Without a clear easy solution, I am leaving this for future development. It is because of this the tests are structured the way they are in the asserts.