title: replace rows in a pandas data frame
body: <p>I want to start with an empty data frame and then add to it one row each time. 
  I can even start with a 0 data frame <code>data=pd.DataFrame(np.zeros(shape=(10,2)),column=["a","b"])</code> and then replace one line each time. </p>
  
  <p>How can I do that?</p>