[ENH] Add Neurobagel vocab graph file

[alyssadai]

Closes #182

Reference for our currently used classes: https://github.com/neurobagel/bagel-cli/blob/main/bagel/models.py

See also #185 for suggested modification of add_data_to_graph.sh to support uploading vocab files to the graph

Checklist

• PR has an interpretable title with a prefix ([ENH], [FIX], [REF], [TST], [CI], [MNT], [INF], [MODEL], [DOC]) (see https://neurobagel.org/contributing/pull_requests for more info)
• PR links to GitHub issue with mention Closes #XXXX
• Tests pass
• Checks pass

For new features:

• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

[coveralls]

Pull Request Test Coverage Report for Build 6279464709

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.754%

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Totals
Change from base Build 6279413011:	0.0%
Covered Lines:	405
Relevant Lines:	406

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💛 - Coveralls

[surchs]

Looks great, thanks @alyssadai 🧑‍🍳