title: Locate first and last non NaN values in a Pandas DataFrame
body: <p>I have a Pandas <code>DataFrame</code> indexed by date. There a number of columns but many columns are only populated for part of the time series. I'd like to find where the first and last values non-<code>NaN</code> values are located so that I can extracts the dates and see how long the time series is for a particular column.</p>
  
  <p>Could somebody point me in the right direction as to how I could go about doing something like this? Thanks in advance.</p>