title: Set a date by weeknumber with carbon
body: <p>I know I can <em>get</em> the week number with <a href="https://github.com/briannesbitt/Carbon" rel="noreferrer">Carbon</a> by calling <code>$my_date-&gt;weekOfYear</code>. Is there a way to <em>set</em> a date by week number? </p>
  
  <p>I mean is there something for initializing a date to the first day of the 3rd week of 2017 year?</p>
  
  <p>I can initialize a date to the start of the year, I think it's a good start but I don't know what to do next:</p>
  
  <pre><code>$a = (new \Carbon\Carbon('2017-01-01'));
  </code></pre>