title: Period of weeks to string with weeks instead of days only
body: <pre><code>var period = Period.ofWeeks(2)
  println("period of two weeks: $period")
  </code></pre>
  
  <p>gives</p>
  
  <pre><code>period of two weeks: P14D
  </code></pre>
  
  <p>Unfortunately for my purpose I need <code>P2W</code> as output, so directly the weeks instead of weeks converted to days. Is there any elegant way to do this, besides building my <code>Period</code> string manually?</p>