Changes to austalk ingest

Most of this is an update to the austalk ingest code to handle additional files that I'm now generating for austalk - there may be annotation files as well as the downsampled and audio files. This code now reads all item rdf files irrespective of names. I'm finding items based on the base item name rather than the -files name as before.  I'm also using a generator over the file-system scan which should be faster.

Generated RDF should be the same with the addition of new metadata if present.

The other change applies to manifest generation which seemed buggy when I tested it.