title: Lemmatization of all pandas cells
body: <p>I have a panda dataframe.
  There is one column, let's name it: 'col'
  Each entry of this column is a list of words. ['word1', 'word2', etc.]</p>
  
  <p>How can I efficiently compute the lemma of all of those words using the nltk library?</p>
  
  <pre><code>import nltk
  nltk.stem.WordNetLemmatizer().lemmatize('word')
  </code></pre>
  
  <p>I want to be able to find a lemma for all words of all cells in one column of a pandas dataset.</p>
  
  <p>My data looks similar to:</p>
  
  <pre><code>import pandas as pd
  data = [[['walked','am','stressed','Fruit']],[['going','gone','walking','riding','running']]]
  df = pd.DataFrame(data,columns=['col'])
  </code></pre>