Author: Cam Spiers (camspiers@gmail.com)
 /**
  * Return an model which has been prepared for classification
  * @return \Camspiers\StatisticalClassifier\Model\ModelInterface
  */
 protected function preparedModel()
 {
     if (!$this->model->isPrepared()) {
         $this->prepareModel();
     }
     return $this->model;
 }