title: Retrive the (index,column) tuple of non nan values
body: <p>Suppose I have this dataframe</p>
  
  <pre><code>pd.DataFrame([[1,np.nan,np.nan],[np.nan,np.nan,np.nan],[np.nan,6,np.nan]])
  </code></pre>
  
  <p>The function should return me the array of tuples:</p>
  
  <pre><code> [(0,0), (2,1)]
  </code></pre>