title: combining pandas dataframe with multiple time series of different length
body: <p>I've got one pandas dataframe df which looks like:</p>
  <pre><code>      date  aaa     date.1  bbb …       date.n   nnn
  31.12.2001  3.0 31.12.2002  6.0 …   31.12.1990  10.2
    1.1.2002  3.1   1.1.2003  6.3 …     1.1.1991  10.1
  </code></pre>
  <p>The time series are of different length. I'd like to have one single column date and the data aaa, bbb, ... ,nnn thereafter. Whenever there is no value for a given date, simply NaN.</p>