public void setStringFieldValue(StringFieldValue stringFieldValue) {
        this.stringFieldValue = stringFieldValue;
    }
Sets the StringFieldValue that this SpanTree belongs to. This is called by
{@link StringFieldValue#setSpanTree(SpanTree)} and there is no need for the user to call this
except in unit tests.

@param stringFieldValue the StringFieldValue that this SpanTree should belong to (might be null to clear the current value)
document/src/main/java/com/yahoo/document/annotation/SpanTree.java