Simplified attachments for v3 #151
Description
As part of v3 I think attachments should conform to the RO-crates/OCFL approach. They should be in a directory called data inside the collection's root directory. They should support versioning using a OCFL directory structure augmented by a dedicated SQLite3 database (to limit the problem of finding the most recent version). Versioning with by via a single increasing integer value. Under the version directory will be a tuple tree (pairtree) holding the attachments associated with a key.
The JSON API needs to support his new disk layout and retrieval.
Versioning should be automatic and follow the approach taken with versioning of dataset JSON objects. The JSON API should work with the "file" input type and continue to follow v3 orientation of read/write as opposed to REST operations.