Determine if advanced rendering is allowed.
By default, advanced rendering will NOT be allowed if the class inheriting from this does not implement a 'renderItems' method.
Overrides FieldPluginBase::allowAdvancedRender
protected function allowAdvancedRender() {
// Note that the advanced render bits also use the presence of
// this method to determine if it needs to render items as a list.
return method_exists($this, 'render_item');
}