public FieldValue setFieldValue(StructuredFieldValue doc, FieldValue fv) {
        FieldValue old = getFieldValue(doc);
        extract.set(doc, (fv == null) ? null : fv.getWrappedValue());
        return old;
    }
This adds an Extractor to the Field that can be used to access the backed value
used in the concrete document types.
@author baldersheim
document/src/main/java/com/yahoo/document/ExtendedField.java