Allow EntityLlinking engines to write entity rankings

Add an option so that EntityLinking engines write the ranking of entities.

The new option will be supported by all EntityLinking Engines. This includes the EntityhubLinking as well as the FST linking engine and can be enabled by setting "enhancer.engines.linking.writeEntityRankings" to true. The default will be "false".

Rankings are expected to represent the relative importance of an Entity within an Vocabulary. Rankings are attached to "fise:EntityAnnotation" instances by using the "fise:entity-ranking" property.

Possible usages are

• by disambiguation engines as an additional imput to the fise:confidence representing how well the mention in the text matches the label of suggested entities
• by clients that want to sort suggestions by popularity and not by confidnece.