public SpanTree annotate(SpanNode node, Annotation annotation) {
        annotation.setSpanNode(node);
        return annotate(annotation);
    }
Adds an Annotation to the internal list of annotations for this SpanTree. Use this when
adding an Annotation that shall annotate a SpanNode. Upon return, Annotation.getSpanNode()
returns the given node.

@param node the node to annotate
@param annotation the Annotation to add
@return this, for chaining
@see com.yahoo.document.annotation.Annotation
document/src/main/java/com/yahoo/document/annotation/SpanTree.java