title: pad rows on a pandas dataframe with zeros till N count
body: <p>Iam loading data via pandas read_csv like so:</p>
  <pre><code>data = pd.read_csv(file_name_item, sep=&quot; &quot;, header=None, usecols=[0,1,2])
  </code></pre>
  <p>which looks like so:</p>
  <pre><code>      0    1    2
  0   257  503   48
  1   167  258   39
  2   172  242   39
  3   172  403   81
  4   180  228   39
  5   183  394  255
  6   192  179   15
  7   192  347  234
  8   192  380  243
  9   192  437  135
  10  211  358  234
  </code></pre>
  <p>I would like to pad this data with zeros till a row count of 256, meaning:</p>
  <pre><code>      0    1    2
  0   157  303   48
  1   167  258   39
  2   172  242   39
  3   172  403   81
  4   180  228   39
  5   183  394  255
  6   192  179   15
  7   192  347  234
  8   192  380  243
  9   192  437  135
  10  211  358  234
  11  0    0    0
  ..  ..   ..   ..
  256 0    0    0
  </code></pre>
  <p>How do I go about doing this? The file could have anything from 1 row to 200 odd rows and I am looking for something generic which pads this dataframe with 0's till 256 rows.</p>
  <p>I am quite new to pandas and could not find any function to do this.</p>