title: Pandas: Forward Fill without Filling trailing NaNs
body: <p>I have a dataframe where each column is a time series of different length. As such, there are missing values both between values in the time series, and at the end of each time series save one column. I would like to fill the missing values between values, but not fill the "trailing" NaNs</p>
  
  <p>Using <code>df = df.fillna(method='ffill')</code> gets me most of the way there, but fills in the trailing NaNs, which I don't want, because where the data ends is actually important to my analysis.</p>
  
  <p>Edit:</p>
  
  <p>I would like to turn this:</p>
  
  <pre><code>            ERICB SS Equity  DCI US Equity  FLEX US Equity
  date
  
  2008-02-14            8.026            NaN             NaN
  2008-02-18              NaN            NaN           1.472
  2008-02-19            8.074            NaN             NaN
  2008-02-22              NaN            NaN           1.532
  2008-02-25            8.062            NaN             NaN
  2008-03-03            8.100            NaN             NaN
  2008-03-06            8.100            NaN           1.955
  2008-03-07            8.100            NaN             NaN
  2010-12-30            5.431            NaN             NaN
  2010-12-31            5.422            NaN             NaN
  2011-01-03            5.422            NaN             NaN
  2011-01-04            5.373            NaN             NaN
  </code></pre>
  
  <p>Into this:</p>
  
  <pre><code>            ERICB SS Equity  DCI US Equity  FLEX US Equity
  date
  
  2008-02-14            8.026            NaN             NaN
  2008-02-18            8.026            NaN           1.472
  2008-02-19            8.074            NaN           1.472
  2008-02-22            8.074            NaN           1.532
  2008-02-25            8.062            NaN           1.532
  2008-03-03            8.100            NaN           1.532
  2008-03-06            8.100            NaN           1.955
  2008-03-07            8.100            NaN             NaN
  2010-12-30            5.431            NaN             NaN
  2010-12-31            5.422            NaN             NaN
  2011-01-03            5.422            NaN             NaN
  2011-01-04            5.373            NaN             NaN
  </code></pre>
  
  <p>So it's forward filled but only when there is some non-null value in the future to fill to, leaving the trailing nulls.</p>