Rename cluster to entity

[martinpopel]

• CorefCluster -> CorefEntity
• mention.cluster -> mention.entity
• cluster.cluster_id -> entity.eid
• cluster.cluster_type -> entity.etype
• for cluster in doc.coref_clusters.values(): -> for entity in doc.coref_entities:
• for cluster_id, cluster in data.coref_clusters.items(): -> for eid, entity in doc.eid_to_entity:
• rename also clusters/cluster -> entities/entity almost everywhere else (variable and parameter names, comments,...)
• new udapi.core.block methods process_coref_mention and process_coref_entity