Replace CopyCrawler in GroupProcessor

[sylvainhalle]

Currently, the duplicate method of the GroupProcessor class uses an object called the CopyCrawler to crawl and pipe processors in the copy like in the original. Crawling could be replaced by merely going through the set of processors of the group (this set is known). There is no added value to a traversal of the processor graph (and this even caused a subtle bug, see a6d17d2.

[sylvainhalle]

CopyCrawler should also be refactored as a class of its own; it still has some uses.