title: Pandas create new date rows and forward fill column values
body: <p>I have a dataframe like this:</p>
  
  <pre><code>id     date       value
   1  12/01/2016      5 
   1  25/02/2016      7 
   1  10/03/2017      13 
   2  02/04/2016      0 
   2  06/07/2016      1 
   2  18/04/2017      6 
  </code></pre>
  
  <p>For each id there is a start date with a value and every few months there is another row with a date and a value.
  I'd like to create a timeseries of each id. So I'd like to insert new rows with the next day (until today), where the value will be forward filled from the previous row.</p>
  
  <p>So the dataframe becomes:</p>
  
  <pre><code>id     date       value
   1  12/01/2016      5 
   1  13/01/2016      5 
   1  14/01/2016      5 
   1  15/01/2016      5
   ...
   1  20/04/2017      13 
   ...
   2  18/04/2017      6 
   2  19/04/2017      6 
   2  20/04/2017      6 
  </code></pre>
  
  <p>Eg answer to a similar question: <a href="https://stackoverflow.com/questions/41591801/appending-datetime-rows-and-forward-filling-data-in-pandas-dataframe">Appending datetime rows and forward filling data in pandas dataframe</a></p>
  
  <p>But my dataframe isn't datetime indexed.</p>
  
  <p>Appreciate any help!</p>