title: Nltk lemmatizers do not recognize the plural form of chemical names
body: <p>So, I must admit, I'm a total noob in nlp, and I have no idea whatsoever about nltk, I'm just trying to use a legacy code left by the previous developer. I need to lemmatize words, mostly from chemical and biotech publications. I generally use WordNetLemmatizer. Most of the time it works.</p>
  
  <pre><code>from nltk.stem import WordNetLemmatizer
  lemmatizer = WordNetLemmatizer()
  lemmatizer.lemmatize('cats')
  </code></pre>
  
  <p>returns cat.</p>
  
  <p>But then I try</p>
  
  <pre><code>lemmatizer.lemmatize('dehydrogenases')
  </code></pre>
  
  <p>it returns 'dehydrogenases'. I want it to return 'dehydrogenase'. How can I do that?</p>