public void register(final VarFactory varFactory) {
        final String id = varFactory.getId();
        if (id != null) {
            map.put(id, varFactory);
        }
    }
Registers a var factory so that it can be linked to group or data
factories by id later on.
stroom-pipeline/src/main/java/stroom/pipeline/xml/converter/ds3/ref/VarFactoryMap.java