title: How to get first day of week when i know week number in ISO 8601
body: <p>I know year and week number for example</p>
  
  <pre><code>year = 2016
  week = 1
  </code></pre>
  
  <p>how to get <strong>first day of week</strong> from this according to <strong>ISO 8601</strong> ? </p>
  
  <p>I have in table <code>datetime2[0]</code> datetype which generate default value <code>2015-12-28</code> but first day of the week should be <code>2016-01-04</code></p>
  
  <p>any idea?</p>