An integer representing a document-level unique cluster of
coreferent entities. In other words, if two entities have the
same CorefClusterIdAnnotation, they are coreferent. This
annotation is typically attached to tokens (CoreLabel).
Returns the type associated with this annotation. This method must
return the same class type as its value type parameter. It feels like
one should be able to get away without this method, but because Java
erases the generic type signature, that info disappears at runtime.