Postprocessing

Allow to configure a postprocessor for a given harvester.

A Postprocessors postprocess method receives a harvested item and can perform any postprocessing action upon it.

This is used e.g: in EOEPCA where the harvested OpenSearch records are looked up and enriched with metadata.

There is currently no default preprocessor, but maybe the CREODIAS one can be put here (or somehow be registered in the registry of preprocessors).