Introduce a Main Topic Field

[mgrani]

A main topic is defined as a topic, that must occur in every result item. The recommender should support such a main topic.

[mgrani]

Please also test whether we need a relaxation strategy, i.e. if a main topic search does not return any result, that we then conduct a relaxed main topic search.

[hziak]

implemented and deployed on the know-demo server
relaxation has to be tested