title: Concatenate cells into a string with separator pandas python
body: <p>Given the following:</p>
  
  <pre><code>df = pd.DataFrame({'col1' : ["a","b"],
              'col2'  : ["ab",np.nan], 'col3' : ["w","e"]})
  </code></pre>
  
  <p>I would like to be able to create a column that joins the content of all three columns into one string, separated by the character "*" while ignoring <code>NaN</code>.</p>
  
  <p>so that I would get something like that for example:</p>
  
  <pre><code>a*ab*w
  b*e
  </code></pre>
  
  <p>Any ideas?</p>
  
  <p>Just realised there were a few additional requirements, I needed the method to work with ints and floats and also to be able to deal with special characters (e.g., letters of Spanish alphabet).</p>