title: Pandas dataframe use columns as rows (melt)
body: <p>I know, this questions has been asked several times, but I didn't manage to build my solution based on those already asked.</p>
  
  <p><strong>DF I have:</strong> </p>
  
  <pre><code>id| country  |  series name   | 2015 | 2016 | 2017
  --+----------+----------------+------+------+------
  0 | saudi    | fertility rate | 1    | 2    | 2   |    
  1 | saudi    | CO2 emissions  | 5    | 10   | 15  | 
  2 | pakistan | fertility rate | 1    | 2    | 1   |
  3 | pakistan | CO2 emissions  | NaN  | NaN  | NaN |
  </code></pre>
  
  <p><strong>DF I want:</strong> </p>
  
  <pre><code>id| country  | year | fertility rate | CO2 emissions 
  --+----------+------+----------------+---------
  0 | saudi    | 2015 |       1        |    5      
  1 | saudi    | 2016 |       2        |    10      
  2 | saudi    | 2017 |       2        |    15      
  3 | pakistan | 2015 |       1        |    NaN
  4 | pakistan | 2016 |       2        |    NaN
  5 | pakistan | 2017 |       1        |    NaN
  </code></pre>
  
  <p>I tried multiple different melt configurations, but somehow I am not able to solve this problem. </p>