Bug/vector index dtype

[ajtritt]

Fix #26

Checklist

• If the current schema version on "master" is a public release:

  • [x] Update the version string in conf.py and namespace.yaml to the next version with the suffix "-alpha"
  • [x] Add a new section in the release notes for the new version with the date "Upcoming"

• [x] Add release notes for the PR

See https://hdmf-common-schema.readthedocs.io/en/latest/software_process.html for more details.

[rly]

Note that this might create some issues (or a lot of warnings) when reading or validating old files that use int values.