Improve context collection for linker

Also -- Changed trait CorefResolved to use a type bound ( type M <: Mention ) rather than a type parameter. This made it possible to know what the mention type was within the linker for the purpose of finding the parent MentionCluster.

Also moved the debug option on OpenIECorefExpandedDocumentExtractor.extract to be a constructor field that defaults to false -- that way all extractors still have the same interface.