Author: Cam Spiers (camspiers@gmail.com)
Inheritance: implements Camspiers\StatisticalClassifier\Normalizer\Token\NormalizerInterface
 /**
  * {@inheritdoc}
  */
 public function normalize(array $tokens)
 {
     return array_map(function ($token) {
         return PorterStemmer::Stem(strtolower($token));
     }, $tokens);
 }