title: Aggregate dataframe with many columns per week
body: <p>I have a large dataframe with many columns and a timestamp <a href="https://i.stack.imgur.com/irULJ.png" rel="nofollow noreferrer"> (see picture)</a>
  What I am trying to do is to aggregate the data per week for all columns. Any suggestions?</p>
  <p>Kind regards,
  Daniel</p>