title: Deprecated rolling window option in OLS from Pandas to Statsmodels
body: <p>as the title suggests, where has the rolling function option in the ols command in Pandas migrated to in statsmodels? I can't seem to find it.
  Pandas tells me doom is in the works:</p>
  
  <pre><code>FutureWarning: The pandas.stats.ols module is deprecated and will be removed in a future version. We refer to external packages like statsmodels, see some examples here: http://statsmodels.sourceforge.net/stable/regression.html
    model = pd.ols(y=series_1, x=mmmm, window=50)
  </code></pre>
  
  <p>in fact, if you do something like:</p>
  
  <pre><code>import statsmodels.api as sm
  
  model = sm.OLS(series_1, mmmm, window=50).fit()
  
  print(model.summary())
  </code></pre>
  
  <p>you get results (window does not impair the running of the code) but you get only the parameters of the regression run on the entire period, not the series of parameters for each of the rolling period it should be supposed to work on.</p>