title: Sum elements with the same index in a new dataframe
body: <p>I have the following dataframe:</p>
  <pre><code>df:
  
             nace_r2   2017   2018  
  NUTS_ID
  
  AT          G-I      5300   2800
  AT1         G-I      2200   1900
  AT2         G-I      2100    900
  AT            J      3900   1500
  AT1           J      2000    700   
  AT2           J      1900    800
  ...
  </code></pre>
  <p>I would like to add all the elements that have the same index (NUTS_ID) and put them in a new dataframe:</p>
  <pre><code>df2:
  
             2017    2018
  NUTS_ID
  
  AT         9200    4300
  AT1        4200    2600
  AT2        4000    1700
  ...
  </code></pre>
  <p>Please note that I have some NaN in my original dataset and I can't simply discard the rows. How could I do this? Thanks in advance for your help.</p>