title: Pandas: interpolation where first and last data point in column is NaN
body: <p>I would like to use the interpolate function, but only between known data values in a pandas DataFrame column. The issue is that the first and last values in the column are often NaN and sometimes it can be many rows before a value is not NaN:</p>
  
  <pre><code>      col 1    col 2
   0    NaN      NaN
   1    NaN      NaN
  ...
  1000   1       NaN
  1001  NaN       1   &lt;-----
  1002   3       NaN  &lt;----- only want to fill in these 'in between value' rows
  1003   4        3
  ...
  3999  NaN      NaN
  4000  NaN      NaN
  </code></pre>
  
  <p>I am tying together a dataset which is updated 'on event' but separately for each column, and is indexed via Timestamp. This means that there are often rows where no data is recorded for some columns, hence a lot of NaNs! </p>