ndx-hed needs a prerelease review

[VisLab]

@rly @oruebel @yarikoptic @saskiad

The ndx-hed is ready for review before release. NOTE: It has to be run with the hedtools 0.7.0, which is on the main branch on github: https://github.com/hed-python and not yet on PyPI.

This release:

1. Creates an extension of LabMetaData called HedLabMetaData, which stores the HED schema for the file and the externally defined definitions. There can be only one of them in an NWBFile.
2. Full supports HED in all aspects of the ndx-events extension, including value and categorical data and definitions.
3. Has round-trip conversion between a BIDS tsv/json file combination and EventsTable.
4. Has full validation equivalent to BIDS. (It accomplishes this by converting to BIDS and validating.) The error messages have been modified (in the yet to be released version of hedtools) to be more platform neutral with respect to where the error is located.

I would greatly appreciate your reviews and suggestions. Is there something that needs to be added or modified? @yarikoptic is there anything that needs to be modified for Dandi -- I'm thinking in particular of the validator interface and tools.

[VisLab]

@rly @oruebel ndx-hed 0.2.0 is ready to be released. Do you have any comments? Do you have a release process or can I go ahead and attempt to release it on pypi.