Refactor the pbcore importer
1. Change from a module with class methods to a module with 2 classes
2. Group all wrapper logic for PbcoreDescDoc into a wrapper class
3. Group all functionality related to ingestion into an Ingester class.
4. user extract method to cut down on mthod complexity
5. Make an 'ingest' method to accompany 'ingest!'
6. Extract relate-pids into methods.