Refactor Entity in BEM:
Fix infinite loop bug.
Subclass BestEntityMention, make findBestEntity take an Entity
Subclass BEM and Refactor.
Add binary serialization for fully extracted documents.
Convert to concrete collections.
Deserialzie to correct type and use NER byte serialziation.
Add test printer for serialized docs.
Add serializable case class ExtractedDocument
Remove unnecessary serialization related code.
Store FB types as List.