Primary Label for Concept Clusters/Graphs

[fmatthies]

When the concept graphs/clusters are stored in Neo4j, a ConceptCluster gets a List<String> as labels (createGraphInNeo4j in ConceptClusterService in top-backend).
Right now, these labels are just generated by sorting the phrases by their document count and taking the three with the most documents. I think, I have a method in the concept-graph-api that sorts phrases by their importance to the graph. The most important phrases should be used as labels. Maybe some form of terminology grounding could be utilized as well to get primary labels.