Create and use ImageReferences dataset data type

[rly]

Summary of changes

• HDMF does not support datasets that have a name and no data type and have dtype reference (see also Document restrictions to datasets without data type inc/def hdmf-dev/hdmf-schema-language#16 ) . It was difficult to modify HDMF to support this rare use case, so this PR modifies the recently added order_of_images dataset to be a new data type.

Checklist

For all schema changes:

• Add release notes for the PR to docs/format/source/format_release_notes.rst.