title: "close all" equivalent for HeatMap plots
body: <p>If you create a bunch of heatmaps:</p>
  
  <pre><code>for i=1:10
      HeatMap(rand(5,5))
  end
  </code></pre>
  
  <p>then you're left with a bunch of open figure windows. Although they're not exactly "Figure" windows, they're "HeatMap" windows (tangential question: why???).</p>
  
  <p>If you have a bunch of normal figures open, you can close them all by typing <code>close all</code>. But this does nothing to HeatMap windows. So... what's the alternative?</p>