title: Combining multiple columns into one column pandas
body: <p>If I have the following dataframe</p>
  
  <pre><code>Animal Animal1 Animal2
  Cat    
  Dog
  Fish   
         Moose
         Horse
         Cow
                Snake
                Bird
                Rat
  </code></pre>
  
  <p>I want to be able to get the desired dataframe:</p>
  
  <pre><code>Animal 
  Cat    
  Dog
  Fish   
  Moose
  Horse
  Cow
  Snake
  Bird
  Rat
  </code></pre>
  
  <p>Please Advise on how to do so in python! Thanks</p>