title: Interpolation on DataFrame in pandas
body: <p>I have a DataFrame, say a volatility surface with index as time and column as strike. How do I do two dimensional interpolation? I can <code>reindex</code> but how do i deal with <code>NaN</code>? I know we can <code>fillna(method='pad')</code> but it is not even linear interpolation. Is there a way we can plug in our own method to do interpolation?</p>