Add "deaccession" and "abort" functions #38
Description
Currently, there is no means to remove an asset from the repository beyond deleting the relevant AIP manually.
There should be a function to remove an asset and record it as deaccessioned in the pymm database.
Use cases for this would be a film that needs to be rescanned or a video that is transferred with different specs.
Similarly, a failed ingest results in some stub AIPs and corresponding database records. There should be a function to select such a stub and scrub all its object and event records (and log entries?) to start from scratch.